Does sendmail send everything to downstream smarthost in "single file"?

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?

Solution 1:

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.