We have had a few times when users report that one or more mailboxes that they used to have delegated permissions to keep returning in Outlook. When we check the delegation on the mailboxes in question the user no longer is listed. Here is the solution we found.
Step 1 – Add the delegation back via PowerShell:
add-mailboxpermission -identity sharedmailbox@domain.com -user user.email@domain.com -accessrights fullaccess -inheritancetype all -automapping $false
Step 2 – Wait for the account to sync between DC’s and Azure
Step 3 – Remove the delegation via PowerShell:
remove-mailboxpermission -identity sharedmailbox@domain.com -user user.email@domain.com -accessrights fullaccess -inheritancetype all
Step 4 – Wait for the account to sync between DC’s and Azure