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Anyone even have a guess?
...
It might help to state which email client was used to send her messages and which email client was used to receive them. That said, it sounds like your gf has an inadvertent space in her mail address.
 
Solved. In case anyone else has this issue...

Mac mail will automatically add your contact info and email address into your signature when setting it up.

Don't delete the email and try to retype it yourself. This is what was causing the problem for us at least. Leave the address and work around it and all will be fine.
 
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