Gmail is supposed to automatically save any sent messages, independently of any mail app doing the sending. Has been for years.
Because of this, AquaMail defaults to not *uploading* sent messages for Gmail accounts, rather it *downloads* them after the sending is complete.
You're the second person in the last few days to have run into this -- maybe they (Google) are changing something?
In any case, please make AquaMail *upload* your Sent:
- Long press the account -> options and folders -> composing and sending
- Enable "Save sent messages"
- Back out and try it
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But please check what StR wrote, first
