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is it possible to group receivers so that there will be only one connection for like 50 messages per receiving domain?
When sendmail runs its queue it does that by default. So if you are sending many messages at once, you need to tell sendmail to just queue them instead of trying immediate delivery.
If sending to hotmail be careful not to trigger their spam filters by sending too many messages at once.
Aslo, I think there is another command limit how many emails sendmail will submit per connection. Using this this with host based priority, multiple queue runners and an increased connection cache can drastically speed up delivery.
If you are looking for performance, consider moving to postfix. On Red Hat boxes it is also supported. Much easier to tune and configure than sendmail.