First of all, Tuffmail.com is not a software. It is an email service. As a service, this is one of the best kept secrets around. The prices are very affordable, you can host your own domain name for email. Actually, you can host many domains without paying additional fees.
The uptime is very impressive. The service is geared towards heavy/business users, though packages are priced from $18 a year.
This service has probably the best spam controls in the market. If you set it up right, you will hardly ever see spam. It also has a ClamAV anti virus installed with an additional Sophos available - virus problems do not exist.
It does support POP, IMAP as well as SMTP so you don't need your ISP for email.
A sample account package includes 1024 MB email storage space (1 Gig) for $28 a year. For a small business one can have a $72 account with double that space and 4 mail boxes.
What I don't like is the control panel for settings. It is a bit cumbersome at first and takes a little while to get used to. Once you are familiar with it, it's very well thought out and functional.