These two Messages features combine to create a terrible UX. When someone starts a group chat in Messages and includes a single non-Apple device, it converts the chat into a group MMS at which point anyone in the group who was added by Apple ID email address starts receiving the messages via email with text messages arriving as text attachments to those emails.
The from/to addresses look like 2345678901@domain with domains such as mypixmessages.com, mms.att.net, mypixmessages.com, icmms1.sun5.lightsurf.net, etc.
It doesn't matter if you have a phone # associated with your Apple ID in addition to an email address. If the person who starts the chat accidentally uses your email address (which Messages makes both very easy to do accidentally and very difficult to notice because Messages hides the actual address behind the contact name) and there's a single non-Apple device in the group, you get to be on the receiving end of a barrage of emails from which you cannot unsubscribe.
When Messages converts a group to MMS, it really needs to make it obvious which recipients are email addresses and which are phone #s... it should either not hide the information behind the contact name and/or for cases where a contact has both a phone # and email addresses, it should favor the phone #.