topic Re: Sendmail timeout in Operating System - Linux

Sendmail timeout

George Doller — Wed, 04 Aug 2010 13:10:04 GMT

I’m pretty new to sendmail and I have a quick question. Can anyone tell me how I can change the time that sendmail waits after it has a failed send before it tries that address again? We currently have an issue where it fails the first time but after it waits the hour it goes through. I’d like to change that time to like 5 minutes. I think it might be Timeout.rcpt=1h????
Thanks

Re: Sendmail timeout

Matti_Kurkela — Wed, 04 Aug 2010 15:31:06 GMT

Sorry, but Timeout.rcpt is not it.

Timeout.rcpt is used when Sendmail is sending a message's recipient list to another mail server and waiting for the other server to accept it. (The "RCPT TO:" step in the SMTP protocol.)

The queue processing interval is controlled by the command-line options when starting Sendmail. A common set of options for the Sendmail daemon is"-bd -q1h": it will make Sendmail both listen for incoming connections (-bd) and process the mail queue in 1-hour intervals (-q1h).

To shorten the queue interval to 5 minutes, the "-q1h" option should be replaced with "-q5m", or simply "-q5" (both are equivalent).

If you specify no time interval at all (i.e. use the command "sendmail -q"), then the command will cause Sendmail to immediately make one extra delivery pass.

When a first email delivery attempt to a particular mail server always fails but subsequent ones will succeed, it might be caused by an anti-spam strategy called "greylisting". It relies on the fact that most mass-mailing programs won't bother retrying.

http://en.wikipedia.org/wiki/Greylisting

MK

Re: Sendmail timeout

George Doller — Wed, 04 Aug 2010 17:46:00 GMT

Thanks for the response. Very helpful.

One more question. When it defers a delivery it basically stops all delivery of that message. Example: if it's sending to user A, B and C. The issue is it sends to A but defers B, C doesn't get delivered either.
Could this be because we're using mailing list and not just listing out the users on the sendmail command line or is it something else?

Thanks

Re: Sendmail timeout

Matti_Kurkela — Thu, 05 Aug 2010 08:22:47 GMT

This would be expected behaviour if recipients B and C are on the same mail server but A is not.

When delivering an outgoing message (assuming that Sendmail is doing a direct delivery, not configured to forward everything to a "smarthost"), Sendmail will first use the DNS to find out the mail servers of each recipient.

For example, if the recipients are:
A@company1.example
B@big.company2.example
C@deptX.company2.example

... then Sendmail will look for MX records for "company1.example", "big.company2.example" and "deptX.company2.example".

(You can check the MX records manually using the "dig" command and the hostname part of the recipient address, e.g. "dig company1.example MX".)

The results of the lookup might be, for each recipient:
A: mail.company1.example
B: smtp.company2.example
C: smtp.company2.example

At this point, Sendmail knows that only one SMTP connection is needed to deliver the message to both recipients B and C. If smtp.company2.example cannot be reached, neither B nor C will receive their messages.

Sendmail is not so dumb it would use two separate SMTP connections to deliver first to B, then to C: back when Sendmail was designed, network bandwidth was *expensive* and optimization like this was important.

MK

Re: Sendmail timeout

George Doller — Thu, 05 Aug 2010 15:28:06 GMT

Thanks again. Any thoughts on why C wouldn't be sent if A, B and C are on different domains?