For AT&T when using the address above with Yahoo email, each message comes in from a different number. I found that if you instead use this MMS address:
AT&T [insert 10-digit number] @mms.att.net
It will show that the message is from the actual email address you use, so all your HG messages will be from one sender. Your texting app may play into this as well. I also make the subject empty ("") to conserve space.
Anyway, someone may find this useful :-)