Qmail

Intro

Written by Dan J. Bernstein, proponent of free speach/software, mathematician in all senses of the word. Solid. Man pages. Almost no source comments. Free use. Free redistribution of the released source (but derivative distributions have to be approved by DJB - security/reliability).

Pro
1. Secure - small amount of isolated SUID and root code; libs.
2. Reliable - Accept only after fsync() of queue files.
3. Reliable - OK on delivery only after fsync() of file.
4. Reliable - Adheres strictly to mail RFCs.
5. Efficient - Multiple parallel message deliveries.
6. Reliable loop detection.
7. User power
8. VERP - Verifiable Envelope Return Paths
9. Easy to configure.
Con

One message per recipient.
Only a drawback under extreme circumstances. In these cases, consider using serialmail (available from DJB's ftp site), sublists, or similar setups.
Adheres strictly to RFCs.
Qmail is correct rather than accepting potential message corruption. This means that it may refuse some messages from non-compliant MTAs, which can send just fine to the majority of older sendmail sites. These MTAs will find life harder and harder - new versions of sendmail are being stricter as well.
Not as proven as sendmail
2 years vs 2 decades. Many security holes found in sendmail, none in qmail. Part of this might be time and size of user base. Of course it may just be that qmail is secure.
Small market share and differnt from sendmail
Most unix professionals are familiar with sendmail, but not with qmail. However, qmail is so much easier to set up. Also, qmail consultants are available via www.qmail.org.
Distribution policy
It's not trivial to make a qmail binary package that gets installed 100% correct and reliably. However, this is what Dan J. Bernstein requires for his approval. The advantage is that once available, the quality is likely to be high. See Dan's WWW site and links for more info.

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