I am having the same issue. My work email and gmail.com both works fine. but if I use my own domain, I can receive emails but not sending out any. Talking to my domain name host today, they said my IP was blacklisted by https://mxtoolbox.com/SuperTool.aspx

I talked to Tmobile and they say there is nothing they can do.

I just rebooted my gateway and the new IP was also blacklisted. so it looks like most Tmobile IPs are no good?

I had this issue as well about a month ago, but fortunately after a couple of reboots of the gateway I was able to get an IP that worked. Haven't had an issue since. It might be a regional issue as well. I recall a discussion on this site awhile ago about users having trouble with Google searches and other services because of blacklisted IPs. 

I appears any IP starts with 172.58 (172.58.*.*) are blacklisted. so there is no way as customers we can do about this.

Since someone mentioned might be regional, I am located in TX. 

I am also experiencing this issue that began out of the blue about 6 weeks ago. Gmail emails are going out without issue but emails from personal domain (hosted via hostgator) are not sending. Switching to ATT hotspot solves problem. The IP being blocked starts with 172.56 (172.56.**.***). Sent a request to Cloudmark to unblock but do not have any high hopes. 

Initially I submitted a request to escalate due to the blocked IP which came back as "no IPs being blocked." The technician at T Mobile I worked with today, had me create another network and try both auto and 2.4ghz just to test and see if that would solve the issue, having a new SSID, but no dice.

Looks like we will have to bail from T-Mobile if this does not get solved in the next few days.