Getting closer - Gmail
After nearly an hour on the line with a really patient Apple support guy, trying everything the two of us could think of, here's the workaround we came up with . . . it appears to have something to do with Gmail and their SMTP. If the gmail account is on, it is high-jacking some of my other POP accounts to have the email appear as if it's coming from Gmail, even if Gmail is not an active SMTP for that account. Once I turn Gmail off, the email sends from its rightful owner.
This is not an issue I'm having on my iPhone, so he thinks it is something within either Apple email or Gmail. For now, I have Gmail on to get that email, but have to turn it off to reply from my critical accounts.
Not the best solution, but workable for now - and better than no solution.