A much more careful search of my logs shows that apart from Verizon's mail servers and SLMail, both of which reply with a 503 after EHLO (instead of 502 like others, but the session continues properly with these machines), all of the other failures associated with this were with the same server, a customer's Exchange 5.5 box that we do spam and virus filtering for. That makes me feel confident that it isn't our problem. I also verified that almost all failures were during business hours, so load on the customer's server or maybe a bad link might be of issue. The failure rate is almost exactly 1%. This should be a kick in the butt for them to upgrade, something that they wanted to do anyway.
Thanks for the help.
Matt
R. Scott Perry wrote:
I got a report of a "503 Bad Sequence" bounce today from a customer and then searched my logs and found many others in there as well. I'm not sure what exactly is going on. It appears that all or most of them occur immediately after the MAIL FROM is issued by my server, and the receiving server is generating the 503 error. I've looked up the RFC and it doesn't seem to indicate a good reason for a 503 error immediately following the MAIL FROM.
To make matters worse, IMail immediately generates a bounce message instead of retrying the message. It would seem that a retry would be in order here.
A 5xx error message is a permanent error, meaning that the mailserver should not retry the message (as opposed to the 4xx messages, which indicate a temporary error, where the mailserver should later re-try the E-mail). The 5xx means that if you re-try the E-mail in the same way later, it isn't going to be accepted.
The "Bad Sequence" implies that the sequence of SMTP commands is wrong (such as sending a DATA before a MAIL FROM). But IMail doesn't send out of order, so it is probably a broken mailserver that you are talking to, that is forgetting part of the transaction.
-Scott
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