When I ceased using Thunderbird as my main email client late last year, I did so because for some mysterious reason it was generating hundreds of sent mail messages every time I used my Android phone to originate or reply to messages .
Back at the mothership, a company account at a hosting company, the sent folder contained hundreds of the same messages with an identical date and time stamp. Curious!
The fix then was to change mail clients on the Android. And moving company mail to a Gmail account made sense.
Thanks to Ryan and Wired