We are aware that Microsoft and Google use this non standards approach. We are also aware that Microsoft change the display name and sending address to those stored in the primary email address of the Microsoft account that the mail is sent from. Very Very unhelpful really.

However to your point the "copies" are not identical. so we leave it to the user to determine if they want both, want them in different folders or just what they want to do.

Ok thanks I didn't look at them carefully enough to know they actually weren't identical.

Evo_Evz said

Ok thanks I didn't look at them carefully enough to know they actually weren't identical.

Probably from your perspective they are identical, but the headers are not, with the google transmission added at their send step that the original Thunderbird sent copy does not have. Not a big thing, but particularly helpful in trying to work out where things are going pear shaped when non delivery is occurring. Having both proves to you it left Google whereas the Thunderbird copy is only proof Thunderbird sent it to Google without error. Having lots of Google sent and no corresponding Thunderbird sent indicates you have mail being sent from another source (perhaps hacked). So it is left to the user to decide. Although if anymore providers start doing it we might have to look at forming a real policy, rather than the ad-hock "we are aware" type of thing I replied to you with.