How Much Can Php's Mail Function Really Handle

I have mixed feelings on the mail() function in php. I've seen it send out amazing numbers of emails and yet I keep reading that it is prone to problems. I'm wondering if someone can put an approximation on what you could/would send out in say 60 seconds. I know that there are tonnes of variables, so i'll try and remove a few:

- server hardware is typical of a cpanel/WHM shared hosting environment
- running apache/linux
- email is say 20kb HTML format
- mail is sent in a php script loop, one by one; alternatively, mail() could be called in clusters of x emails - like send 5 emails at once
- there are 400 recipients