eM Client manages multiple e-mail accounts well, but its free version is limited by how many accounts it can support. It makes up for it with abundant style, a relatively easy setup, and great features, though.
This program behaves like a typical e-mail manager, akin to Outlook or Thunderbird. In addition to letting you manage multiple e-mail accounts, eM Client will organize your calendar, meetings, contacts, and more for you so you don't have to think about it. You can get right to work with this client by inserting your e-mail and letting it slowly populate all of the rest of the information it needs. It has a widget for most of the popular e-mail providers and even lets you import from other clients like Outlook. It usually works without a hitch, but it may give you some difficultly if you have two-step verification on the account you're trying to set up. The program only accepts two accounts if you use the free version, so you'll need to upgrade to Pro if you want unlimited accounts. Either way, the program sports a gorgeous layout that supports color-coding and multiple themes. There's even a built-in chat that supports ICQ, Jabber, Yahoo, and most of the other popular IM clients.
eM Client is so loaded with features that it's easy to forget it only supports two accounts. While this could be a burden for some, others won't have a problem. If you're happy with that limitation, this program rewards you with a stylish layout and plenty of control over your accounts.