Eudora

Eudora is one of the preferred email clients for users who need to keep track of several different email accounts at the same time.

It's really easy to use. Back in the day, it pioneered options such as the one to import files from other email clients.

Eudora includes many different anti-spam filters, a customizable toolbar, an address book, and an inbox window. It also supports postcards as well as drag-and-drop technology.

With this excellent application, you can add plugins to record voices (PureVoice), use spellcheck, and much more.

In short, it's an excellent application for managing your email accounts.