New answers tagged imap
You are most likely not sending the message twice, but are seeing the sent message in mail, and then again from the server. If you uncheck the "store messages on the server" box under mailbox behaviors, you will only be saving the sent messages to mail.
I recently tested Mail in a new User account to find all the files that are modified after adding an account to Mail. These following files contain account info. I can't say with certainty that you will need to reset all files in order to reset your account. Remember to log out/in and empty the trash before resetting. Make sure you have backup if needed. ...
If using the default mail app, Settings > Mail, Contacts, Calendars > (pick account) > (click account again) > Outgoing mail server You should be able to configure which SMTP server will be used for the account.
My hypothesis is that what you observe is happening because you are setting your IMAP account not to have all the message for offline consumption, but only headers. If you had it set to full messages, you would be decreasing your space even more, because the IMAP metadata needs to be kept in a database, apart from the full messages.
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