MTA Solutions: qmail vs sendmail vs Postfix vs Exim

Each of these four widely-used MTAs have broadly similar features. All of them can handle large amounts of mail; can interact with databases in many formats; have an extensive knowledge of the many SMTP variants in use; are not trivially exploitable; have the source code available in a free manner; have third-party documentation available, etc etc. There are some assumptions implicit in what follows. If you are looking for a product that presents an administrative interface and performance results similar to Microsoft Exchange or Lotus Notes, this document is not for you.