WARNING: [5021-108-12-00000000] Unable to open free/busy for user 'Invalid Exchange address format (4301579)

WARNING: [5021-108-12-00000000] Unable to open free/busy for user 'Invalid Exchange address format.'

While migrating Resources this warning may occur if the Free/Busy is not working correctly on the target Resource mailboxes.

1. Login to the target resource mailbox
2. Right-click on the Calendar folder | Properties
3. Select Permissions Tab
4. Select OK

The above steps force the mail client to re-populate permissions on the Local Free/Busy folder. Please then test whether the error still occurs. If still an issue, please try the following:

1. Launch Outlook with the /cleanfreebusy switch
2. Connect to the target resource mailbox
3. Verify whether the users can now schedule appointments with the resource mailbox

See the Microsoft article on Outlook switches here:

https://support.office.com/en-ca/article/Command-line-switches-for-Outlook-2013-079164cd-4ef5-4178-b235-441737deb3a6>