Sysadmin

okay, so you've got 10 employees with MS licensing. They have e-mail addresses through their microsoft licensing, correct? Those e-mails go through an exchange server? (whether on-premises or exchange online, right?)

The other 30 employees have e-mail using...what, exactly? (What kind of mail server?)

Because that Microsoft doc is typically used to set up stuff like scan-to-email for copiers and although you can do other things with relays, knowing the precise setup (without the exact domain names, just use domain1.com, domain2.com or whatever) is important here in order to get the MX records and any relays pointing at the servers where the addresses reside.

554 does seem to indicate a configuration or permissions issue, but to go down that route, I'd need a little more info about the non-microsoft side of things.