Email to Text Message not being delivered from NetSuite for some AT&T users

We have a workflow that sends a simple Email but to a Mobile number in the form: [email protected] or [email protected] Up until the 15th of last month (February, 2021) these emails were received as Text messages by their intended recipients without issue.

We have since received notifications from at least two (2) users who are no long able to receive those email messages as Text messages if they're sent as an email from NetSuite. Other emails sent directly to the recipients phone number, outside NetSuite, are received without issue.

I have confirmed that the messages are being sent by adding in my own phone number as a BCC email recipient in the same format indicated above. I have received the messages successfully even though the recipient in the "To" box does not receive the message.