Login to Office 365 Admin center, go to Setup – Domain, and add a new domain. You can enter e.g. tst.axr.at. You can also do the following steps with a complete new domain that you only use for test accounts.
If you added the domain, select Exchange as only online service and wait for the upcoming screen with the updated DNS entries.
Press verify and thats it! First step done!
Open the exchange admin center, go to "mail flow" and to accepted domains.
This is necessary because without setting it, all emails without an existing mail address will be dropped by the system. You can send mails to [email protected], but they will be blocked before a rule is executed!
Press save and accept (means ignore) the warning.
Do the following: "Redirect the message to…" – select your user or a shared mailbox (where all testers have access).
Save it and try it out.