How to avoid "Delivery Failure" when forwarding emails to phone-number@vtext.com

Starting in summer 2022, I repeatedly get the following error message when I tried to forward emails to my phone number as a SMS message. The email-forwarding action is triggered by a filter I set in my Gmail account.

I was following this post and used phone-number@vtext.com as the forwarding address. Yet, when I start to see this sort of Delivery Failure messages, the speed of the SMS-forwarding also went very slow - years ago, SMS may even beat my mail client on the phone. Now, SMS may or may not be delivered, a good number of hours after the message is received.