qmail is used on the Linux platform by
Plesk and other server administration systems.
To allow qmail to send up to 400 concurrent emails, as opposed to the default 10 or 20, set up a file in your qmail
control directory:
/var/qmail/control/concurrencyremote
In this file, have a single line containing the following number:
400
This change will considerably increase your outgoing email speed!
You can also increase delivery to local email, though, this isn't nearly as important.
In the file:
/var/qmail/control/concurrencylocal
Enter this:
100
Don't forget to restart qmail
/etc/rc.d/init.d/qmail restart