Description: This solution provides information on resolving the SMTP response of 5.7.60
An error occurred sending an email to 'firstname.lastname@example.org'.
Error message: 'Mailbox unavailable. The server response was: 5.7.60
SMTP; Client does not have permissions to send as this sender'.
The SMTP relay does not allow the 'sent on behalf of address' to be different than the SMTP username. You will need to use the same address for the 'sent on behalf of address' that is used for the SMTP username. Please see the below link from Microsoft for more information.
5.7.60 SMTP; Client does not have permissions to send as this sender:
For more detailed information regarding MailOmatic for Accounting, please refer to the User Guide for MailOmatic for Accounting