It's just a computer (or program, depending on context). It can do whatever you want it to.
If I want to write/modify a mail server that watches video feeds from 6 different beaches and only bothers accepting mail when beaches 2, 3 and 5 are empty and beaches 1, 4, and 6 have 500 people, nothing is stopping me. It's stupid and a waste of time, but it's a computer. It can run arbitrary code.
That's ignoring that if you read what he wants, it would be a client to the actual recipient mail server and only needs to actually serve the web interface so that he can access his email from various browsers.