A word of caution to those planning a migration to Exchange Online while using the hybrid method and have more than one email address policy applied in the on premise organization. Please review your email address policies before moving any mailboxes for the following potential issue(s).
In this example, we have 2 email address policies: one custom policy and the default policy.
For the default policy, the default criteria and conditions cannot be modified…
… and have a single custom SMTP address of “first name initial and last name” (i.e. firstname.lastname@example.org). Let’s assume this is an older configured policy that is not really in use.
For the custom (newer) email address policy, the criteria is set to specific recipient types “Users with Exchange mailboxes”, with no conditions being set…
… and two custom SMTP addresses: first name underscore last name (i.e. email@example.com) and included the SMTP address from the default policy of “first name initial and last name” (i.e. firstname.lastname@example.org).
NOTE: The default reply address is set to %email@example.com in the custom policy compared to %firstname.lastname@example.org in the default policy.
Now, when a mailbox is migrated to Exchange Online the Default Policy gets applied to the mailbox instead of the custom policy. As a result, the default reply address is set to email@example.com instead of what might have been originally intended. In this case, firstname.lastname@example.org is still present as an email address but it is not set as the default reply address.
The issue is with how the custom policy is configured and to whom the specific email address policy is applied when an Exchange mailbox leaves the organization. Because the custom policy is set to apply to “Users with Exchange mailboxes” only and not “All recipient types,” when the mailbox is moved to Exchange Online the mailbox is no longer an Exchange mailbox in the eyes of the on premise organization. And because the custom policy is applied to only Exchange mailboxes (on premise) the default policy updates the default reply address because the custom policy no longer applies. Therefore, the incorrect email address is set as the default reply address in Exchange Online which is different than what was originally intended for the migrated mailbox.
To resolve the issue, the custom policy can be set for all recipient types or the default policy updated to include what is needed and then delete the custom policy. Finally, wait for AAD Connect to sync the changes to Office 365.
Good luck and have fun!