One of our customers was getting the below error and it took ages to find a solution so I thought I would post it here.

Unexpected Exchange mailbox Server error: Server: [server.domain] User: [useremail] HTTP status code: [503]. Verify that the Exchange mailbox Server is working correctly.

For more information, see Help and Support Center at

This is how I fixed the issue:

  1. Open IIS
  2. Right click Default-Website
  3. Click Properties
  4. Click advanced
  5. Review sites. most likely you will see host headers and ip address.
  6. Click add
  7. IP address = (all unassigned)
  8. TCP Port = 80
  9. Host Header Value = (Blank)
  10. Click OK
  11. delete the entry with the host headers and ip address assignerd.

This should resolve the issue please comment if you have any issues doing this.