Teams Email is causing issues....

Hi - I'm a novice when it comes to Teams admin, so here goes.

People we invite to meetings are getting error messages. One example: ...hmmm can't post screenshotes here?

Ok... the error messages:

The problem appears to be: Recipient email address is possibly incorrect. 

Additional Information:

5.1.1 The email address

550 Recipient address rejected: user unknown in relay recipient table

Any suggestions on how to turn the Teams email off? I dont use it. 

Thanks!

Shayne