Conclusion: messages are not simple!!!

But folders have many problems as well:

• Mbox: format of separator line, escaping Froms in the body, size of the folder, no sub-folder support.
• MH/Maildir/POP are slow to search.
• Maildir is stateless with flags in filename.
• POP3/IMAP connect to host (connection lost?)

Storage formats are usually not well described, and have small differences between implementations. Supporting the various folder formats is severely hindered by this.