A client of mine recently moved from on-premise Exchange to Office 365 for email.
They occasionally send emails to the text addresses of a small group of their own employees.
Since the move, they have had intermittent issues with SMS messages being rejected as spam by T-Mobile.
I see that a recent thread on this subject was closed E-mail to SMS being blocked due to suspected SPAM
srickar You were helping people with similar issues and I've sent you a connection request hoping you can help with mine also.
From what I gather from the previous thread. 10 digit @tmomail are no longer allowed. Companies now use shortcodes, like 99338 for the Kane Show radio show. Your company needs to look into updating to use the shortcode service, in which those who receive messages from shortcodes, must opt-in to send to and receive messages from these types of texts.
Manage your T-Mobile account