Been trying to figure it out, for me (hopefully you too) it just seems to be the stock messenger that comes with the S9. I downloaded Google's Android Messages from the Play Store and I was able to send MMS through that app. For now I've made Android Messages my default app for messaging as I try to find a way to fix Samsung's messenger.
I have the same issue and I am with ATT. Both of my phones are NEW S9+ activated 3/30 and issue showed up same day . I began contacting ATT 4/2 about issue. They sent me to Samsuung 4/15. IT is now 5/31 an I have invested over 40 hours with Samsung techs to resolve the issue with no resolution. I have spent over 73 hours with ATT ProTech with resolution. Noone offered to replace phones both companies blame each other. Samsung set me to a service center 45 miles from my home and the service center stated they would need the phonw for a minimum of 3 days up to 28 days. No Loaner - NOT ACCEPTABLE!
Let me know if you find a resolution.
Maybe I will have to become an APPLE user for the first time.
Oh wow, I'm having this same issue. I haven't reached out to any techs or anyone else. I was hoping to find an answer here. Think I'm going to just download Google messenger. Thanks