Guest users enabled in Global Address List, but 'Email Address' field blank

Question

How do I populate the 'Email Address' field for guest users in the Outlook (classic) Address Book?

I changed -HiddenFromAddressListsEnabled to $false for a series of Guest Mail Users; they now show in the GAL; however, the 'Email Address' field is blank when searching/browsing the directory.

Functionally, the list entries operate: if selected as a reciepent an email can be sent and is delivered. This makes sense as when right-clicking and viewing 'Properties' there is an entry for SMTP:[guestemailaddress], but as the 'Email Address' field is blank it looks like it's broken and is likely to confuse end users.

Answer 1

Outlook uses the value of the WindowsEmailAddress property to populate this field, which I'm guessing has a null value on said guest user objects. You cannot edit how Outlook handles this, but you can populate said field on the affected users:

Set-MailUser ******@domain.com -WindowsEmailAddress ******@externaldomain.com

Keep in mind that updating the value of WindowsEmailAddress will also update the value of PrimarySMTPAddress.