When you click the send button in Postmaster Express, the PE Mailer is launched.
The Mailer uses threads or processes to send out your mail faster. The default setting for the number of threads the Mailer will maintain is 3. For many clients and computers, this is the optimal setting.
The Mailer will maintain X amount of threads at all times. This does not mean that only 3 threads will be opened. Each thread created will que and send out 15 emails. Thus, if you are trying to send out 180 emails, it will take a total of 12 threads to send out the email. The PE Mailer will maintain 3 threads at all times (if this is what you have the amount of threads set to) until all the email is sent out.
Why are threads important?
Instead of only trying to send out 1 email at a time, we can send out X emails at a time. X would theoretically be the number of threads the Mailer is maintaining. The default setting is 3 so the Mailer is sending out 3 emails at a time instead of just one. This means that Postmaster Express can send email out much faster.
How do I know what the best setting is for the amount of Threads to maintain?
If the speed of mailing is not a factor though, it's best to send slower into Gmail, Yahoo, etc.