I've been running the company email system on Notes for a long time. Since I was a Notes admin it seemed like the easy choice, but unfortunately over time Notes became less and less effective at stopping the web nasties from pouring in, eventually getting to the point where I was clearing out (manually) over 50 mails a day, and that's after the thunderbird spam filter cleared out another few hundred on top of that. This was all brought to a head with the purchase of the iPhone. The iPhone connects directly to the IMAP mail server so if Thunderbird is not running on my laptop, no junk gets cleaned. The iPhone became instantly useless for mail.
Now I know my way around Linux and had briefly set up and run a postfix server but it's a world of pain.I wanted something that just worked with the minimum of fuss: POP, IMAP, SMTP, webmail, with virus scanning and anti-spam server side.
Then I stumbled across qmail.
I downloaded a premade virtual machine from http://www.qmtiso.com/ and gave it a run. Seemed to fit the bill, but trying to load it into my ESXi server was a world of pain as it was built with IDE drivers and ESXi runs SCSI. Back to the site, I purchased a copy of their latest ISO (20 bucks!) and installed it straight into ESXi. Setting up the domains and accounts was a snap, then following the setup guide the rest was running perfectly. Took maybe an hour all up which is pretty amazing, since I've burned many hours in the past trying to set up opensource servers using unintelligible config files.
If you're looking for a new mail system, I recommend qmail. Oh and my level of spam has dropped to around 3 a day. :-)