[SOLVED] Sendmail not waiting for 250 response

rjkfsm · 08-31-2018, 07:52 AM

Hello,

I am using 4 ea sendmail servers to act as proxy servers to filter out abuse from the NDRs & FBLs being returned. The NDRs are being processed by the 20 Selligent StrongMail servers that sent the original message. The FBLs get forwarded to another server that processes them.

The issue is that sendmail is not waiting for the 250 response after sending the RCPT TO which is causing the Selligent server to reset the connection and cause the emails to be deferred & queued.

Code:

220 mta-RRRF.cluster2.<company>.net - ready at Fri, 31 Aug 2018 07:42:49 -0500 for server 32737
>>> EHLO mail301.<company>.net
250-mta-RRRF.cluster2.<company>.net
250-PIPELINING
250-SIZE 10240000
250 8BITMIME
>>> MAIL From:<fbl@bounce.mailstream.senderscore.net> SIZE=19447 BODY=8BITMIME
250 Ok
>>> RCPT To:<complaint@cluster3.<company>.net>
>>> DATA
250 Ok
354 send the mail data, end with .
>>> .

... Deferred: Connection reset by [74.xx.xx.xx]

I have used tcpdump to capture the packets and confirm the issue.

So, the question is: How do I fix this? How do I get Sendmail to wait for the 250 response before sending the DATA?

Russ

rjkfsm · 08-31-2018, 02:14 PM

Update: I have been informed that RFC 2920 states that when a server advertises PIPELINING (which the Selligent server is doing), the client MTA does not have to wait for a 250 response before sending the next command. I have thrown this at Selligent and they now acknowledge a bug with "no timeline" for resolution. They advise, "In the meantime, unfortunately, the only workaround for the moment is to avoid batching of commands (i.e. not making use of pipelining)."

Is there a way to do this in sendmail? Disable that feature?

Russ

rjkfsm · 08-31-2018, 02:33 PM

Update2: The only way to disable command batching in Sendmail is to change the source code and re-compile. (It's not a runtime option)

Russ

rjkfsm · 08-31-2018, 08:18 PM

Update3: Selligent support had me alter a mapping file so that the PIPELINING doesn't come up anymore after the EHLO and so Sendmail does not try to batch commands.