Please remember to activate POP3/IMAP in your account settings
before setting up your application.
Apple Mail synchronizes with your GMX mailbox.
In the menu bar, click Mail and then Add Account....
Select Other Mail Account... and then click
Enter your name, email address and password and click Sign In.
Choose the account type IMAP and enter
imap.gmx.com as server for
incoming email. Enter mail.gmx.com as the server for outgoing email.
Then click Sign In.