Some smtp servers are set to automatically save a copy of each outgoing message. So maybe 'inbox.com' does just that (gmail does so by default, fastmail can do it but won't by default, most ISP smtp servers won't...).
In AquaMail, if you have configured your inbox.com account to save sent messages, then you could end up with two copies of the same sent message:
- a first copy saved by the smtp server itself (later downloaded to your device upon 'sent items' folder sync)
- a second copy saved by AquaMail (uploaded to the 'sent items' on the server once the message has been sent)
This is what I would try:
- Try disabling saving sent messages for this account (long press on account name, Options and folders, Composing and sending, Save to Sent folder after sending)
- Send a message with the account
- Check your sent items for this account (the message should not be there)
- Sync your sent items (the message should then appear if it was saved by the smtp server)