Mail.app unable to login to 1&1 mail account after upgrading to macOS High Sierra
I had the same problem. The solution was in the Mail app itself to go to Preferences, select Accounts, and to enter the complete email myname@myisp.de as user name for my IMAP account.

I also had to change the port for IMAP to 993 and for SMTP to 465 (for secure connections via SSL.)