How to remove a block from a hotmail account?

My spectrum account suddenly began bouncing a friend's email sent from his hotmail address with error code AUP#In-1010, suspicious activity. He uses hotmail on the desktop or mobile (android) web. The blocked messages were sent from his phone.

Spectrum's description of error code 1010:
https://www.spectrum.net/support/internet/understanding-email-error-codes

The likelihood of my 80-year-old friend, who hardly knows how to use his cell phone, engaging in suspicious email activity is so remote as to be laughable. That's why I'm posting this instead of him.

Is someone forging his address? Could it be a misconfiguration of a Microsoft server?

And is there anything he can do to remove the block? (I've already suggested he change his very weak password.)

Thanks for any help.