SMTP not relaying SMS text messages

Heya guys, thanks in advance for any assistance on this. I'm getting a weird behavior that just started happening last month. I have a MS Server 2019 (on premise VM) that is setup as a SMTP relay. The relay sends it up to o365. The SMTP software used is hmailServer. To test, i use a simple command line utility called blat.exe. When testing, it works great to a regular email address, and it comes across fine. But when i attempt to use an email address in SMS text format, it doesn't work. The logs in the SMTP server shows that it is being sent off without errors.

e.g. 123456789@txt.att.net or 123456789@vtext.com

In Outlook, i am able to send an email directly to those SMS email addresses.

In hmailServer interface, i do not see anything that would be blocking to those types of addresses, and went so far as to whitelist them during the troubleshooting process; of course to no avail.

The fact that it works with one type of email address, and not another type throws the whole thing off.

I was going to ask if there's a setting in o365 that prevents SMS email addresses from being used, but as mentioned, it is working when sending a direct email from Outlook bypassing the SMTP server. Perhaps I'm not thinking granular enough? Is there something in o365 that prevents SMS text via SMTP?

Thanks for all the help

https://192168o1.com.br/
https://19216801login.com.br/
https://logintplink.com.br/