Pegasus Mail is an email client with a very long history (the first release appeared way back in 1990).
And there's a sign of this in the program's compatibility. The developer's site still has a version available for Windows 3.1, for instance, not exactly a high priority operating system for most other email clients.
Pegasus Mail has of course developed a great deal over its 20+ years, though, so it does have plenty of features to offer.
There's support for all the main mail protocols (SMTP, POP3, IMAP4, LDAP, PH, with SSL support everywhere for security), for instance. A capable message editor helps you write and format plain and HTML messages, while a built-in spam filter helps to keep your inbox clean.
And the program comes with powerful mail filters; a mail merge option; spell-checker; plugin support, and the list goes on.
Pegasus Mail still won't be for everyone; it's essentially a one-man operation and updates are relatively rare. There is a fair amount of power here, though, and if perhaps you're having problems running other email clients on an old PC then it may be worth a look.
It may be old and updated only occasionally, but Pegasus Mail still has more features than a lot of the free competition and may be worth a look.