The Bat! Home Download
One of the best e-mail clients. It is also very advanced: multiple users support, advanced filtering method for e-mails (usage of filters and macros), HTML e-mails, PGP coding is only a part of its possibilities. It also have ready e-mail templates, adress book and an option of tracking incoming messages - MailTicker. A comparison of Proffesional and Home version is available here.
Polish version is available here.