Strange issue today, synced a bunch of users from on prem to Exchange which was successful.
Then the user was missing his default SMTP address on EoL but the SMTP was there on prem:
EoL, no COM:
EX prem, has COM
First thing to check, is the domain setup as a accepted domain in EoL (should be as the migration would of failed otherwise).
Next you can “override” the sync process by using the following EoL cmdlet:
Set-Mailbox email@example.com -WindowsEmailAddress firstname.lastname@example.org