Best practices for migrating to O365 Exchange e-mail

We are running Tools 8.59.17, FSCM, and HCM. We are sending PeopleSoft-generated e-mails through a local SMTP server, but we are migrating all e-mail to Office 365 Exchange. Our current SMTP server does not require authentication, but O365 does, and it uses that to count licensed users. We would like to check your experiences and best practices for the following issues:

1. We currently use a fake sender e-mail addresses for each Tuxedo domain -- 24-30 total. This makes it easy to identify the domain that issues a system alert e-mail such as batch process notifications. But because of license cost, we would like to reduce this to as few as possible -- ideally one user to send ALL system notifications. Is there an easy alternate way to identify the Tuxedo domain in such messages?