I recently had a similar problem.. A client noticed that no e-mail he sent would be saved in the sent-items folder, despite the correct configuration.
Deeper analysis at the filesystem level revealed corruption in the sent-items maildir (no "cur" and no "new" folders). Creating them with appropriate permissions, or deleting+creating the folder in IMP, would do the trick.
As for why this corruption occured, still no clue.
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