Thunderbird

Thunderbird is a free, open source, cross-platform e-mail and news client. The project strategy is modeled after Mozilla Firefox.

Advantages compared with the most popular web e-mail application:

(a) Excellent Mail-Filter Management, filters can be sorted; 
(b) E-Mails can be saved as txt-files; 
(c) Pictures/screenshots can be pasted directly from the clipboard in new mails without storing+loading again from harddisk.

Automated signature features, if using more than one sender e-mail addresses.