I don't understand your description. It seems to me that you contradicted yourself.

Is it spaced out when you send it? This sentence suggests it gets there OK: When she gets it the mail is formatted as I would expect. I even had her just bounce back an email without touching it. the formatting is fine.

So when does it go wrong? Or can you be more precise about what "formatted as I would expect" means?

What if you addressed it to someone else, composed the message then changed it to this particular recipient?

What mail client is this correspondent using? Apple's Mail does some odd things…

I tried to post an example the the forum software keeps reformatting my words to make the example useless.

When I send her a message, every time I hit CR it looks like I've hit it twice. However, when she gets the mail there is only 1 CR (which is the expected behaviour). When she sends my mail back to me there is only 1 CR.

So the problem is only in composing the message.

That is unbelievably weird!

Try a screenshot.

So, her name/address has to be in To, or Cc? What if you add it later? Does it switch to weird mode then?

Yeah, unbelievably weird.

It also happens when I compose the message before adding a recipient.

I tried with a couple other people, works fine.

To be honest I was hoping there was a setting I couldn't find that I could set/unset to solve this. As there evidently isn't let's assume my install got borked somehow and, as it's not really hurting anything, quit wasting time on it.

If it bugs me enough I can always backup my email and do the reinstall two step.