When you sync users to Exchange Online, the primary SMTP address is shown incorrectly as email@example.com or the secondary SMTP address is missing. Depending on your configuration, this may cause mail routing issues and generate non-delivery reports (NDRs) for these recipients.
For example, you receive an NDR that contains the following error message:
554 5.4.14 Hop Count exceeded – possible mail loop ATTR34
This occurs by design when you have one or more unverified domains in Office 365.