I also had a Situation last time, when i could not send Mail Alerts to a customer, because the Sender and Recipient are the same and the Office365 Exchange Server from the Client does not accept Mails from another Source using the Maildomian configured with Office365.
So i had to do a Workaround an configure an Distribution list for the customer on our (external) Mailserver an send the Mail Alert to this list, which then Forwards it to the final recipient, an Office 365 Exchange Mailbox.
It would be much easier, if Sender and Recipient could be different.
Then we could use e.g. Alarm-Backup@DummyDomain.com
as Sender and the real Mail-Address of the Customer as Recipient.
That´s what we do with many other products and this works fine.
I think it should be a very easy exercise for you Veeam Guys to have a separate Sender and Recipient.
I count on you Veeam Guys. You do a great Job, also with B&R