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CentOS 5.x / SendMail

Hi All,

Normally when I connect to my SendMail server, I see the SMTP banner greeting almost instantly.

I've noticed though, that when the primary DNS server is unavailable for the SendMail server to access, the SMTP banner greeting can take upwards of 10-15 seconds to appear. I assume that this probably has something to do with SendMail trying to perform a reverse DNS lookup, failing to receive a response, and then trying the second resolver (and/or giving up on the query).

Assuming this is the case, is this configurable? I reviewed the Tweaking Configuration guide ( and saw several options pertaining to resolver timeouts but they all seemed specific to OUTBOUND delivery FROM sendmail. This delay is happening for INBOUND delivery TO sendmail.

Any thoughts? Thanks.

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Yes it is because of the DNS issues. This is why you should keep your DNS up and running, and why there is a secondary DNS server.

To resolve this you need to tweak the DNS and not the Sendmail. Resolve the cause and not the symptom.

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… and since this won't actually cause problems I wouldn't worry about it. – MikeyB Oct 31 '11 at 20:26

Comment out FEATURE('accept_unresolvable_domains')dnl

in your file and rebuild/restart sendmail.

share|improve this answer
Hi @David Scwartz... I have two questions: 1) Wouldn't this have the opposite effect and block more? 2) From what I can tell, this comes into play while SendMail is looking at the MAIL FROM data. The delay I'm seeing is right at the beginning -- waiting for the SMTP banner to come up. Would this feature apply? – Mike B Oct 31 '11 at 22:23
The delay on startup is but one of your problems. – David Schwartz Oct 31 '11 at 22:32

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