Yes. It's because the Messages UI conflates iMessage groups (blue bubbles) with SMS/MMS/RCS groups (green bubbles), combined with the fact that you can iMessage an Apple user using either their phone number or any email address they have enabled for iMessaging.
So say you start a group message to Apple users. You add type their names and don't pay attention to whether Messages is using their phone number or email address to add them to the group. (Even worse, after you're done typing their name, Messages only shows the name so you can't even tell how they've been added w/o taping their name again.)
Now, when it's an iMessage group (blue bubbles), it doesn't matter how you added them.
But as soon as you add a non-Apple user, it becomes an SMS/MMS/RCS group (green bubbles).
Guess what happens to the Apple users who were added by email address? That's right, they get emails.
There's no indication whatsoever to anyone in the group that this is happening. And as a member of the group, you can't fix how you were added. The message group has to be abandoned and a new group needs to be created using only phone numbers.
It's a terrible UI. I've been dealing with it for years because originally I used Google Voice and there's both Apple and Android users in my extended family. But even after I finally ported my number over to my phone, my extended family still sometimes adds me to SMS/MMS/RCS groups using my email address.
(Why don't we just use WhatsApp you ask? Oh, we use that too. With varying degrees of technical literacy, we end up using all the things.)
Could it be that the person has the email address saved as the default contact method in their contacts?
For a bit of time I had no cell service, but people could use Messages to send to my email address for my iCloud account. Once cell service was restored to the device, messages from one person always came addressed to the email while everyone else reverted to the phone number. I just assumed that this person's contacts listing for me was updated, or possibly even a separate contact using just my email???? I never figured it out/confirmed it either
They talked about it on the accidental tech podcast years ago when someone emailed him with the solution to this problem which one of the hosts was having.
I’ve tried searching but I’m unable to find which episode it was since search engines don’t work anymore.
So here’s what I think it might be based on what little I remember. When messages you’re sending are going to the wrong address, I think what you…
1. Tap the contact pic at the top of the iMessage thread
2. On the first row of buttons, tap the far right “info” button with a generic contact poster icon
3. Tap the “message” link
4. All their iMessage addresses will pop up, choose the one you want to use
I think that might “move” the thread to the right place.
I know in your case it sounds like it’s the other party who would need to do this. If this doesn’t actually do anything or create a second thread, I’m sorry. I know there’s a way I just don’t remember it if it’s not the above.
