I am currently looking at possible MTA's for a rather large mail cluster. The basic plan is a netapp filer serving an NFS mounted mail spool to as many Sun Netra's as necessary. Two things are critical: reliable performance and reliable perforamnce. Looking at around 5000 users, but they are very heavy users. The biggest problem with our current distributed sendmail setup is that the mbox file format overloads disk IO on any configuration we can reasonably obtain (as well as being a 5 year old design.)

Obviously, maildir is the answer. But which MTA?

Sendmail - proven reliable, and is basically what we're using now. qmail - proven reliable, but not so much as sendmail. Also it is quirky and seemingly unmaintained. Courier - looks sexy, but is it reliable?

Thus, the question: Has anyone used courier in a high availability environment with large users? If so, any comments?

Thanks,

Patrick