Things do break, so all alarms need periodic testing, whether they consist of email and/or something else.

Whether it breaks because a relay has corroded due to years of service in a chlorine-enriched environment, or because O365 is down, or because O365 tightened up on DMARC requirements: It needs tested.

And it's not a binary choice. This isn't the World Cup, the Super Bowl, or Highlander. There can be more than one.

The alarm system can wear a belt, and suspenders. It can send emails, and it can flash a bright light, and it can also sound an audible alarm.