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

Return to top

HVU Blocks

Return to top

DNS Blocks

Return to top

RLY Blocks

Return to top

DYN Blocks

CON Blocks

Return to top

DMARC Blocks

Return to top

MFF Blocks

Return to top

Mailer-Daemon Errors

An error message from AOL's e-mail delivery subsystem ( 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. 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 <> was undeliverable.
The reason for the problem:
5.1.0 - Unknown address error 550-'MAILBOX NOT FOUND'

Reporting-MTA: dns;
Final-Recipient: rfc822;
Action: failed
Status: 5.0.0 (permanent failure)
Remote-MTA: dns; []
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: 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.

554 5.2.1 (DMARC)
This message failed DMARC Evaluation for an AOL Domain. For more information, please visit the postmaster blog.

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.

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:
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 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.

Return to top

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.

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 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:

Return to top