Exchange Online Default Email Address Policy

So this was interesting.  I had a user that was failing to create an Exchange Online Mailbox, due to a duplicate tenant address (tenantname.onmicrosoft.com), which us mortal people have zero control over.

After a week of playing I raised a call with Microsoft to see what the hell was going on.

To cut a long story short, the issue was resolved by itself.  “When product group started the troubleshooting mailbox was already created.”  Yeah right! As if by magic after a week it sorts itself .. sorry not convinced!

Anyway, it turns out (according to PSS), that the Default Email Address Policy on Exchange Online actually does nothing, and updating it from the default email template, has zero effect on users!  Which seams a bit odd, but hey!?!

The other interesting thing, is that if two users share the same UPN prefix like bob@domain1.com and bob@domain2.com there will be a conflict because both should get bob@tenantname.onmicrosoft.com as an alias in Office 365.

We have over 100k synced objects and over 600 domains.  So we should have seen this issue before .. but we didn’t.

Anyway, this is where the duplicate attribute resiliency feature of AAD Connect should come in to play, to make sure the attributes are unique. https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnectsyncservice-duplicate-attribute-resiliency.

However, for the user I had an issue with, this didn’t happen!

The expected behavior should be:

1st user to be synced / provisioned:
UPN / primary SMTP: bob@domain1.com
Secondary SMTP / cloud alias: bob@tenantname.onmicrosoft.com
Remote routing address: whatever is set via Exchange console (using the tenantname.mail.onmicrosoft.com domain)

2nd user to be synced / provisioned:
UPN / primary SMTP: bob@domain2.com
Secondary SMTP / cloud alias: bob1234@tenantname.onmicrosoft.com
Remote routing address: whatever is set via Exchange console (using the tenantname.mail.onmicrosoft.com domain)

So, only the IT gods know what happened, and it shouldn’t have happened in the first place, but welcome to the occasional oddness of cloud computing and Office 365.

UPDATE#1: Resolution from Microsoft: Attribute resiliency feature action generated unique onmicrosoft.com smtp address, but unfortunately with significant delay.

Laters