(Applies to: Epicenter Server)

Symptom: You are running a hybrid Exchange environment with O365 and on-premises. You receive an error similar to this:

06/29/17 06:14:04 [Global Address List] Skipping /o=NT5/ou=00000000000000000000000000000000/cn=74DEB4546735E24E93BF2E7CF92C6E69. No server found for mailbox. Possible causes: 1. This user is an external contact.  2. This user is not mailbox enabled.

Cause: 

User was created directly in O365, which means that the user was not migrated to O365 and so the user does not have a mailboxguid on the remote user object which is created on the on-premise installation to route mails, handle group memberships and such.

Solution:

Copy the MailboxGUID from O365 to the user object on the on-premise installation.