I just replied to a similar question on another of Apple's forum pages, but as so many of us have had this question, thought I'd paste in my solution here for everyone's benefit -------
I think I FINALLY solved the same problem -- after looking for solutions online for weeks.
I have two accounts, one for personal mail and one to interact with institutions/organizations/charities with. I noticed that almost every time I emailed my husband something from my photos (and some other things) the Apple email changed my default "send from" address (my personal one) to my institutional email, which I didn't want to happen. I sometimes forgot to individually change the "send" to my personal one. This also happened with a few other personal correspondents, getting their mail all mixed in with my two email addresses.
Yet, once in a while when I sent my husband AND others something from photos, I noticed the "from" address remained my personal (default) one. This led me to my solution. Today, I decided to delete my husband's email addresses from my contacts AND from every single "recent" group. When I re-entered his email addresses into my contacts list, I sent him a couple of test messages (forwarding photos as my test case), they came from the default (personal) address.
I hope this helps others! I live in a city with a big Apple store, but the folks there didn't know what to do about the problem when I brought it to their attention months ago.