iPhone fails to send some text, “Not Delivered” message

ArmandoWyo wrote:

My iPhone SE fails to send some, not all texts. Happens only to some recipients and these were friends who up until a short time ago I could send and receive texts. I don’t get their text either. With these contacts “Not Delivered” also happens on iMessage on MBP.

can’t find any pattern to which recipients I can no longer text. Perhaps they don't have iPhones? At least some of them don't.

I checked: I have not been blocked by them.

Start here If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support