It is important to note that 1 and 1 has multiple hosting email plans. Normally, users can log in to Match My Email using email address and email password. However, with 1and1 hosted Exchange plan for either 1and1.co.uk or 1and1.com, a user must use a special username to login via IMAP. (Examples below are for 1and1.com. Please substitute imap.exchange.1and1.co.uk if you are using the 1and1.co.uk hosting.)
IMAP SETTINGS DURING ACCOUNT SIGNUP
IMAP SETTINGS on IMPORT TAB
Cloud Admins adding Domain information for a 1and1 Hosted Exchange server should set Authentication to Username Only.
For assistance or questions, please contact firstname.lastname@example.org.