|Transient Delivery Attempts||Failure Time||Details||Recipient|
|Attempted Delivery to: default-non-local-route||Thursday, Jan 03, 2013 03:48:55 PM CET||451 4.4.1 [internal] connection was not attempted: domain is marked down (too many connection failures)||[email protected]|
When too many connection attempts to the downstream mail server for a domain fails; that domain is marked as "down" by the Messaging Gateway for a short period of time (usually five minutes).
This is to conserve resources when, after repeated connection failures, the destination server is unlikely to be reachable.
During the period of time where a domain is marked down, SMG queues messages for that domain rather than immediately attempting delivery.
Once the domain is no longer marked as down, delivery of new messages will be attempted immediately and delivery of queued messages will be attempted at the next queue cycle (usually within fifteen minutes).
This is usually the result of the destination mail server being unreachable for reasons such as:
In case of SMG not using DNS MX records for mail routing but rather a static route in Protocols->Domains, check that the remote server is reachable via telnet from the command line as shown below and confirm that firewall rules are not preventing mail delivery to the destination IP:
smg> telnet mail.domain.com 25
Connected to 188.8.131.52.
Escape character is '^]'.
220 mail.domain.com ESMTP Mail
221 2.0.0 Bye
Connection closed by foreign host.
Flushing the delivery queue will not cause messages for a "down" domain to be re-attempted until the timeout has expired and the destination server is reachable.