Delivery Error Messages and What they Mean
There are a lot of different error messages that you will see in the log file. Some come from Hurricane MTA Server while others come from remote servers. This article attempts to document some of the more common errors.
Permanent Errors - These are failures that will cause the message to fail permanently.
Below are standard errors that mean the account does not exist.
554 delivery error: dd This user doesn't have an account 550 5.1.1 Mailbox <address> does not exist 500 No such user 553 5.3.0 <address>... Addressee unknown 550 Requested action not taken: mailbox unavailable
The below error is a permanent failure and means that the DNS server returned an error specifically saying that the domain does not exists.
Name service error for baddomain.com Type=A: Host not found
Temporary Errors - These are errors that will keep the message in queue to be retried at a later time.
The below error means that the MTA was able to get the MX records for the domain but it was unable to connect to any of them.
Could not connect after trying all mx records. type=mx
The below error means that no MX records were returned for the domain. Standard SMTP practice is to try to connect to the A record of the domain that has no MX records. This has also failed.
Could not connect to server. Type=A
Greylisting - a method of defending e-mail users against spam.
A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will, after a delay, try again and if sufficient time has elapsed, the email will be accepted. If the mail is from a spammer it will probably not be retried since a spammer goes through thousands of email addresses and typically cannot afford the time delay to retry. Below are examples of what greylisting errors look like.
450 4.7.1 <address>: Recipient address rejected: Greylisted 450 4.7.1 <address>: Recipient address rejected: Policy Rejection 451 Message temporarily deferred - 
Below is a standard defer message that is not a greylisting error. This occurs often when the remote server is too busy to process the request.
451 Resources temporarily not available - Please try again later [#4.16.5]
Deliverability Error Messages - Hurricane MTA server has delivery rules that are use to control the sending process to domains that have specific delivery restrictions and limitations. These restrictions include maximum messages per connection, maximum connections, and maximum simultaneous connections, to name a few. Sometimes when Hurricane MTA Server is trying to enforce these rules it may need to intentionally defer a message. That may happen in a case where, for example, Gmail.com hosts many different domain names yet all the domains point to the same MX records. In this situation you may have exceeded the maximum number of connections to gmail.com's server. If Hurricane MTA server finds another domain that goes to the same record, it will need to wait for a connection to become available before it sends the message. To show that the MTA is doing this it will add the log entries below.
421 4.4.5 System rule canceled connection. 421 4.4.5 Delivery rule action forced defer.