MailDomain field is always blank with mail co-existing using SMTP method. Please review the attached screenshot to this article.
WORKAROUND
1- On "Specify Notes to Exchange mail forwarding" screen specify mail domain, mail system, and calendar domain
2- Do not select "Set mail Forwarding on Mail Forwarding address"
3- Click next until you see the summary with [blank]
4- Come back to "Specify Notes to EXchange mail forwarding" screen
5- Re-provide mail domain, mail system, calendar domain
6- Select "set mail forwarding"
7- Select build address from:
8- De-select "set mail forwarding"
9- Make sure it is greyed out and build address from: is still selected but greyed out
10-Go to summary screen and verify you do not see [blank]
STATUS
Waiting for fix in a future release of Notes Migrator for Exchange.
For additional status updates or information on this please contact your account manager and reference the Change Request # ST104785.