I recently downloaded a great new mail app for Mac called Airmail. You can get it on the Mac App Store.
Setting it up with an Office365.com company account is a little finicky. Here are the settings I used to get things working.
IMAP
Username your full email address name@company.com
Incoming server: outlook.office365.com
Port: 993, Full TLS
PLAIN
SMTP
Sending server: smtp.office365.com
Port: 587, TLS
Username: your full email address name@company.com
Password: your password
LOGIN
Then start enjoying your great new mail app!