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CentOS 5.x / SendMail 8.14.x

I have a SendMail server that delivers all outbound mail to a downstream smarthost in the cloud:

define(`SMART_HOST',`relay:[example.blah.com]')dnl

example.blah.com has multiple A records and MTAs available to accept connections.

In the event that multiple emails need to go out at once, will sendmail establish multiple connections to the downstream smarthost? I did a few sanity checks and netstat suggests that sendmail will try using multiple simultaneous connections when things get busy.

If that's the case, what are the default rules governing that? How many connections will sendmail establish to a downstream source? When it's establishing these connections will it use the same A record that it resolved before? Or is it possible that these other connections could go to other A records/hosts?

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  • Are you ready to accept delays (up to a few minutes) in email transfer to help reuse SMTP connections?
    – AnFi
    Jul 25, 2014 at 19:29
  • @AndrzejA.Filip In this case, I just want to know what SendMail does by default and if this is configurable at all. I agree that changing this behavior could cause a bottleneck.
    – Mike B
    Jul 26, 2014 at 2:23
  • There is HUGE number of options affecting it. Short answer requires more specific description of "problem to fix", your question is too general IMHO.
    – AnFi
    Jul 26, 2014 at 18:08
  • @AndrzejA.Filip It's more of a general question. The system is performing fine. It sounds to me like the answer is "By default, sendmail will use more than one connection when there is a need to deliver multiple messages to a downstream smarthost. The specifics of how many connections it decides to use depend on a moderately complex algorithm so it's difficult to say exactly how many connections will be used."
    – Mike B
    Jul 26, 2014 at 18:20

1 Answer 1

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By default sendmail uses background DeliveryMode - sendmail forks another process for message delivery after accepting any message. It means separate SMTP connection per each "first time delivery attempt" unless some limits are hit at sending or receiving server. Sendmail attempts to reuse SMTP connections during processing of queued messages. There is HUGE number of options to change/fine tune it e.g. SingleThreadDelivery, different DeliveryMode, expensive mailers (no at once delivery), queue groups and persistent queue runners.

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