Hi @Ramos simtnt ，
Does this issue only affect this particular user?
Are you able to see the mailbox from the Exchange Online EAC > Recipients > Mailboxes? If not, is it available under Recipients > Contacts?
For current situation, you can have a try by performing a hard match by using GUID and see if it can help. For more details about the steps, you could refer to: Soft (SMTP) vs. Hard (immutableID) matching with Azure AD Connect.
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