421 4.2.0 sender rejected AUP#SNDR

  • 18 May 2022
  • 2 replies


I manage a few mail servers for my employer. We noticed that we are being blocked by T-Mobile ( We send a lot of messages to our customers' SMS gateway addresses daily.

As of today, all of our messages have started. On the servers we saw bouncing back with 421 sender rejected AUP#SNDR messages.

Is there any way you could whitelist our mail server IP addresses?  I know you are making some changes to fight spam, but it is hurting our clients who want to get notified on new property listings.

I can email the logs to you at an appropriate email address or complete a whitelist form if you would like, along with a list of our mail server IP addresses.

Please assist us with this issue.

2 replies

Userlevel 6
Badge +13


might be some needed info in here..




Having the same problem..

I am forwarding a few messages from my gmail email account to my T-Mobile number so I can get text messages of those emails..

The last few days the T-Mobile service has been returning this error..

At first it was just having trouble and rejecting delivery.. Now its outright blocking the messages..

What is going on? I’m just a regular T-Mobile user with a personal phone number.. I don’t have a business account or anything.. I just want to forward some messages from my gmail to my phone as text messages.. What do I need to do to make that happen again?

It was working just fine last week, and for the last several months since I set it up.. But the past week its been failing, and now I’m getting this error.

Can someone help me with this please?

Thanks in advance!