mailprogam choice, experience

It also depends on your server. Mercury32 documentation/FAQs say that it can handle about 8,000 per hour on simple PC hardware. It also depends on the size of the email. Larger ones take longer. The receiving email servers also matter - are they slow?

Unless you are sending out tens or hundreds of thousands of emails, you probably don't need to worry. If you are using your web hosting account to send emails, then you may need to worry if you are sending any more than a few hundred or a couple thousand emails.

50 or 100 emails a day is nothing. Don't worry about it.