http://postmaster.aol.com/Postmaster.Errors.php

AOL SMTP Error Messages

When there is a problem delivering your message to its destination you will receive an error message from our system, either in your mail server's logs, or as a bounce-back. The following information will give you more details on the meaning of the error message and the steps you can take to solve the problem. For a list of non spam-policy-specific error messages (example: 550 user not accepting mail from this sender) please visit

Other Mailer-Daemon Errors.
The codes are listed in categories based on the first part of the error message or the section that comes before the colon. Click the appropriate link above to find your error message. (example; 554 HVU:B1, click on the HVU link and then navigate to your error message.) If you need help interpreting the information in the e-mail and finding the error message included, please read Other Mailer-Daemon Errors on this page, or check out interpreting e-mail headers.

RTR Blocks

554 RTR:BB
AOL works with many ISPs to maintain lists of dynamic and residential IP addresses. Per our E-mail Guidelines, we do not accept mail from these addresses. If you are blocked with RTR:BB and have a fully qualified domain name (FQDN), open a support request to lift an RTR:BB block. If you do not have a FQDN, please have your ISP create one for you and then open a support request.

554 RTR:CH
A computer or computer(s) on your network may be compromised and is sending mail that matches the signature of known spam. Ensure you have a fully qualified domain name, get approved for a complaint feedback loop, scan your system for compromises and secure any issues before opening a support request.

554 RTR:BG
This error indicates you are sending email from an IP address not yet delegated or allocated. Talk to your ISP or hosting company about this problem before opening a support request.

554 RTR:RD
The reverse DNS of your IP Address is typical of a dynamic IP Address and/or has generated AOL Member complaints. Ensure you have a fully qualified domain name and get approved for a complaint feedback loop before opening a support request.

rDNS must be in the form of a fully-qualified domain name. rDNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. rDNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between domain and IP address.
rDNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect, and should therefore should be changed to reflect a fully-qualified domain name with standard reverse DNS.

554 RTR:SC
This error message is indicative of a block due to successive dynamic blocks issued against your IP address. Get approved for a complaint feedback loop before opening a support request.

554 RTRU
AOL uses the Spamhaus PBL to block mail from dynamic and residential IP addresses. Per our E-mail Guidelines, we do not accept mail from such addresses. If you believe your IP is listed in error, please contact your ISP directly and have them update their listing with the PBL. If...
your ISP reports that the IP is correctly listed in the PBL, and that you should be able to send mail from it, or
you were recently assigned IPs, have changed the rDNS on them, and allowed 48-72 hours for propagation time...and you are still getting the error, please open a support request.

554 RTR:GE
This a transient error indicating a technical issue on AOL's side. Please wait 24 hours and re-try your mail.

554 RTR:BL
This error message indicates that a permanent block has been placed against your IP due to poor IP reputation. Apply for a complaint feedback loop before opening a support request.
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HVU Blocks

554 HVU:B1
There is at least one URL or domain in your e-mail that is generating substantial complaints from AOL members. Resolution will require opening a support request.

421 HVU:B2
There is at least one URL or domain in your e-mail that is generating substantial complaints from AOL members. Resolution will require opening a support request.
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DNS Blocks

554 DNS:B1
The reverse DNS of your IP Address is typical of a dynamic/residential IP Address. Please have it updated to something unique like mail.example.com, or use the mail servers your ISP provides for residential customer traffic. If the rDNS has been updated and the problem persists, please open a support request.

rDNS must be in the form of a fully-qualified domain name. rDNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. rDNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between domain and IP address.
rDNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect, and should therefore should be changed to reflect a fully-qualified domain name with standard reverse DNS.

554 DNS:B2
The reverse DNS of your IP Address is typical of a dynamic/residential IP Address and has generated AOL Member complaints. Please have it updated to something unique like mail.example.com, or use the mail servers your ISP provides for residential customer traffic. If the rDNS has been updated and the problem persists, please open a support request.
rDNS must be in the form of a fully-qualified domain name. rDNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. rDNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between domain and IP address.
rDNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect, and should therefore should be changed to reflect a fully-qualified domain name with standard reverse DNS.

421 DNS:NR
The Reverse DNS lookup for your IP address is failing. This could be a transient issue. Confirm the IP that sends your mail. Then check the rDNS of that IP using our troubleshooting tools. If it passes, please wait 24 hours and re-try before opening a support request.
rDNS must be in the form of a fully-qualified domain name. rDNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. rDNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between domain and IP address.
rDNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect, and should therefore should be changed to reflect a fully-qualified domain name with standard reverse DNS.


RLY Blocks

554 RLY:B1
This error message is a dynamic block on our system. Dynamic blocks are placed on an IP address when the IP's statistics break our threshold. These are automated blocks that are removed by the system within 24 hours once the complaints are again below the threshold.

554 RLY:B2
This error message indicates that a hard block has been placed against your IP address due to poor IP reputation. Resolution will require opening a support request once the IP reputation can be improved.

421 RLY:B3
The IP address you are sending from has been temporarily rate limited due poor reputation. Please visit this page for tips to improve your IP reputation

554 RLY:BD
This error message indicates that a hard block has been placed against your email because it has triggered too many anti-spam flags. This block is not against the IP. It targets specific types of email.

421 RLY:CH
A computer or computer(s) on your network may be compromised and is sending mail that matches the signature of known spam. Ensure you have a fully qualified domain name, apply for a complaint feedback loop, as well as scanning your systems and securing any compromises before opening a support request.

554 RLY:CH
A computer or computer(s) on your network may be compromised and is sending mail that matches the signature of known spam. Ensure you have a fully qualified domain name, apply for a complaint feedback loop, as well as scanning your systems and securing any compromises before opening a support request.

421 RLY:CH2
Your IP address has generated AOL member complaints and your mail system may be compromised due to a virus or other security related issue. Ensure you have a fully qualified domain name, apply for a complaint feedback loop, as well as scanning your systems and securing any compromises before opening a support request.

554 RLY:CS4
This error message indicates that your email has been identified as coming from a server or web site that may have a compromised script or program installed. Ensure you have a fully qualified domain name, apply for a complaint feedback loop, as well as scanning your systems and securing any compromises before opening a support request.

554 RLY:IR
The reverse DNS of your IP Address is typical of a dynamic/residential IP Address and has a high percentage of invalid recipient attempts. Ensure you have a fully qualified domain name, apply for a complaint feedback loop, as well as scanning your systems and securing any compromises before opening a support request.

421 RLY:NW
The IP address you are sending from has been temporarily rate limited because the IP is new or has been idle for a long time. Please allow 24-48 hours and continue to send mail normally. It takes a little time to determine your IP's reputation.

421 RLY:SN
This error indicates you are sending email using a disallowed AOL.COM screenname as your FROM or REPLY-TO address, or as one of AOL's affiliates from an unauthorized IP address. Example: Billing@aol.com

554 RLY:SN
This error indicates you are sending email using a disallowed AOL.COM screenname as your FROM or REPLY-TO address, or as one of AOL's affiliates from an unauthorized IP address. Example: Billing@aol.com
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DYN Blocks

421 DYN:T1
The IP address you are sending from has been temporarily rate limited due to lack of whitelisting, unexpected changes in volume, or poor IP reputation.

421 DYN:T2
The IP address you are sending from has been asked to delay email delivery to AOL due to a temporary issue with our mail system. This will be corrected by AOL and no changes should be necessary on your part. If the issue persists, feel free to open a support request with our postmasters.
CON Blocks

554 CON:B1
The IP address has been blocked due to a spike in unfavorable e-mail statistics.

421 CON:B1
The IP address has been tempfailed due to a spike in unfavorable e-mail statistics.
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Mailer-Daemon Errors

An error message from AOL's e-mail delivery subsystem (mailer-daemon@aol.com) indicates that there was a problem in delivering your message to its destination. When you receive an error from our mail system, your message will be returned to you along with an outline of the mail delivery problem. See also: AOL Error Messages. Due to technological and security limitations, some of the listed error messages may be sent in a bounce message and not during the mail transaction process.

The pertinent part of the bounce will look like this:

The following message to <exampleuser@aol.com> was undeliverable.

The reason for the problem:
5.1.0 - Unknown address error 550-'MAILBOX NOT FOUND'

Reporting-MTA: dns; mail.example.com
Final-Recipient: rfc822;exampleuser@aol.com
Action: failed
Status: 5.0.0 (permanent failure)
Remote-MTA: dns; [1.2.3.3]

Diagnostic-Code: smtp; 5.1.0 - Unknown address error 550-'MAILBOX NOT FOUND' (delivery attempts: 0)

The bounce will also contain your original e-mail that we are returning to you as undelivered.
Here are some common errors with brief explanations as they pertain to our mail system.

Note: the following is not an error, but the AOL SMTP welcome banner. It will display when an SMTP connection is made to any of our relays, whether the connection IP has rDNS or not:

220-rly-dd08.mx.aol.com ESMTP mail_relay_in-dd08.6; Mon, 12 Oct 2009 15:31:48 -0400
220-America Online (AOL) and its affiliated companies do not
220- authorize the use of its proprietary computers and computer
220- networks to accept, transmit, or distribute unsolicited bulk
220- e-mail sent from the internet. Effective immediately: AOL
220- may no longer accept connections from IP addresses which
220 have no reverse-DNS (PTR record) assigned.
250 OK

This is not an error. This indicates your email has been accepted and queued for delivery.
552 Mailbox full

This error indicates that the AOL Member's mailbox is full. The AOL member will need to free up additional space before they can receive email.

552 Message exceeds maximum fixed size
This indicates that the mail sent was larger than AOL currently allows. The largest piece of e-mail that an AOL member can accept from or sent to the Internet is 16 megabytes. This includes the message text, headers and the attachment combined. These sizes can not be changed.

550 "username" Is Not Accepting Mail From This Sender
This error indicates that the AOL Member has configured his account, possibly inadvertently, to only accept mail from certain addresses and/or domains on the Internet. The member will have to change their Mail Controls in order to receive mail from you. AOL Postmasters can not override these Member defined settings. We respect Member's privacy and their ability to control their own mail. AOL Postmasters cannot forward the mail for you. AOL members that have set this rule accidentally can view instructions on how to change the setting, or contact AOL Member Services.

550 Mailbox not found
This error indicates that the AOL Member no longer exists on AOL or the address is misspelled.

550 Mailbox not found

500 5.1.1 : Recipient address rejected: aol.com
Senders using scripts to remove unknown users automatically, should be looking for both the new error code and the old one.
550 Access denied
This error indicates that your site has been blocked from sending e-mail to AOL. Contact your e-mail administrator for assistance.

550 Sender domain not found in DNS
These errors indicate that there are DNS resolution problems somewhere between your domain and AOL. This usually means that AOL is unable to resolve your domain name in DNS before accepting that mail for delivery.

550 Requested action not taken: DNS FAILURE
These errors indicate that there are DNS resolution problems somewhere between your domain and AOL. This usually means that AOL is unable to resolve your domain name in DNS before accepting that mail for delivery. Contact your e-mail administrator for assistance. The network administrator for the site may have to modify their DNS records.

550 Service unavailable or SMTP error from remote mailer after initial connection
The information presently available to AOL indicates this server is being used to transmit unsolicited e-mail to AOL. Based on AOL's Unsolicited Bulk E-mail policy, AOL cannot accept further e-mail transactions from this server or domain. Please have your ISP/ASP or server admin visit http://postmaster.info.aol.com for more information.
If you are receiving the above bounce message, please check our open relay search page and ensure your mail server is not open for relaying, and have your systems administrator check for open proxies as well.

452 Requested action not taken
This error indicates your mail server or ISP's mail server has insufficient disk space . Please contact your ISP or system administrator.

500 Syntax error, command unrecognized
This may include errors such as command line too long, or if you are statically assigning one of our IPs. Please contact your ISP or system administrator to help resolve this issue.

What is Reverse DNS and what requirements does AOL have for it?

Reverse DNS is a way of associating an IP address with its hostname. The reverse DNS identifier is contained in the PTR portion of the IP Zone File. The IP Zone File contains all the different ways that your IP and domain name can be associated; each association serves a different need.
AOL requires that all connecting Mail Transfer Agents have established reverse DNS, regardless of whether it matches the domain.

Reverse DNS must be in the form of a fully-qualified domain name. Reverse DNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. Reverse DNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between an IP address and its associated domain.

Reverse DNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect, and should therefore should be changed to reflect a fully-qualified domain name with standard MTA reverse DNS. [Example: mail.aol.com]

Any host over the Internet is supposed to have a valid reverse DNS (PTR Resource Record) declared, as required by RFC 1033: Domain administrators operations guide, section Adding a host:

Adding a host:
To add a new host to your zone files:
Edit the appropriate zone file for the domain the host is in.
Add an entry for each address of the host.
Optionally add CNAME, HINFO, WKS, and MX records.
Add the reverse IN-ADDR entry for each host address in the appropriate zone files for each network the host in on.

While it is technically possible to declare multiple PTR records for a given IP address, this is generally useless. In fact, multiple PTR records may confuse some programs which may end up picking one randomly among the different values and ignoring the others, resulting in unpredictable results. For this reason, we recommend that one single PTR record be declared on each public IP address.
Ensure this label is declared in the direct DNS zone and points back at the same IP address, otherwise such a PTR record may be deemed spoofed and result in denying access. In other words, ensure PTR and A records match and are consistent, as recommended by RFC 1912, Common DNS Operational and Configuration Errors, paragraph 2.1:

2.1 Inconsistent, Missing, or Bad Data
Every Internet-reachable host should have a name. The consequences of this are becoming more and more obvious. Many services available on the Internet will not talk to you if you aren't correctly registered in the DNS.

Make sure your PTR and A records match. For every IP address, there should be a matching PTR record in the in-addr.arpa domain. If a host is multi-homed, (more than one IP address) make sure that all IP addresses have a corresponding PTR record (not just the first one).
Failure to have matching PTR and A records can cause loss of Internet services similar to not being registered in the DNS at all. Also, PTR records must point back to a valid A record, not a alias defined by a CNAME. It is highly recommended that you use some software which automates this checking, or generate your DNS data from a database which automatically creates consistent data.

Verifying DNS conformance
Ensure that your PTR and A records are visible by the rest of the world over the Internet, as sometimes they appear fine internally within your organization, but are not propagated over the Internet due to a delegation failure. Use one of the many free web-based tools available over the Internet to verify your reverse DNS records as they are seen by the rest of the world:

http://remote.12dt.com/
http://www.dnsgoodies.com/

AOL reverse DNS Tool