Bersoft Private Mail

Bersoft Private Mail (BPM) is an email program that runs on most Windows versions, from XP to Windows 10. Its goal is to keep email messages private, both storing and (optionally) sending them as encrypted messages. BPM stores mail locally encrypted with AES-256 cipher and can send encrypted email using a PGP like RSA cipher. It is portable, supports IMAP and POP3 email accounts, has SPAM filtering, and a powerfull search function.

Limitations:

30-day trial, limited to 512-bit RSA keys