Professional, well that's a little vague
Hi Ben!
It seems more large-style business, with MANY employees using email tend to lean towards the more well defined, well tested Sendmail. There's plenty of books on it, documentation galore, and it comes default with many distros.
Qmail is a great alternative, it's more secure, gives more control of mail delivery to the users, and has many tools to help you work with it (such as IMAP servers).
The choice is really yours (the admins) on which to use, both are very capable, both are used professionally.
An excellent, similar mail server to Sendmail is Postfix. This is sometimes considered more secure while still mainitaining the flexibility that the standard mbox mailservers provide. Another plus is that Postfix is able to easily use the Maildir format that Qmail currently employs. It's considered MUCH more secure than the standard /var/mail/spool method of mail storage.
HTH
My personal suggestion, Postfix. Qmail can be quite a daunting task if you are not familiar with how it works. However postfix is very well self-documented, has the familar conf file you are probably use to, and is capable of both mailbox delivery methods. It's kind of the "Swiss Army Knife" of mail servers, it will do whatever you want, and is easy to see what you need to do to make it work.
Cool
