Duplicate entries in sent mail
My sent folder displays duplicate outgoing email in Thunderbird with every message I send from my Fastmail email account. It does not do this with two other accounts that Thunderbird handles.
Alle antwurden (1)
If, as it appears, fastmail automatically makes copies of every sent message, you should uncheck that option in Tools/Options/Account Settings, select the fastmail account in the left pane, then Copies & Folders 'Place a copy in...'.