Are you talking an email CLIENT or an email SERVER, the two are different.
Neither really need to be "commercial" per se.
For the server part, install any of the qmail packages available on freshmeat. They allow a good number of things there.
For the client part, try horde for webmail, or kmail for non. I've always been torn between kmail and evolution, both are extremely great products, neither charge anything
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Re: Good commercial e-mail software packages for UNIX?
Sorry, just to clarify, looking for something on the server side... I understand the OSS options clearly, I am looking for something a little more "small business oriented" in terms of installation and management. Ensim, plesk, etc. are really not viable options when you're explaining to a secretary the difference between POP and SMTP.
I used to recommend CommuniGate frequently, but with the new license structure, its not really geared towards small business. I was lucky enough to have purchased a 2000 seat license a few months before the went to the 4.2 license scheme where groupware was rolled in. My 2000 seat license for email cost under $2000, and supposedly included unlimited upgrades, but once they released 4.2, groupware was no longer optional and the price for the same license would be closer to $20k.
I guess I made out pretty good, but now upgrades/support is priced at 19-24% of the new purchase price per year, so at some point I'll probably be looking to migrate away from it because its tough to justify $4000 a year for support when your just using it for pop/smtp
As a product, its a fantastic mailserver. Reliable, stable, feature rich, and easy to admin.