We need protocol logs to see what's going wrong. Details on your IMAP account would be greatly appreciated too if they are custom (IMAP and SMTP servers).
- Go into Preferences/Settings, Shake your iPhone. "Enabled" should show.
- Scroll down and activate 'Debug'.
- Run sparrow until your problem re-appear.
- Go back in Settings / Debug.
- Select 'Send Logs', write a quick description of your issue in the dedicated text field and you're done.