I completely agree with you that the mbox is far from a perfect solution. The lock problem on the same file was a problem to code, and it's not perfect ;)

On the other hand, as you suggest, there is maildir. No locks, but the drawback is the high number of files that your file system and Java have to deal with. Also, I initially wrote a mbox support because most system I have seen runs with it. It's a good idea to provide a maildir support, I've planned it (I need to test how Java handles large number of files first, and studdy a little how qmail works)

For the pop/imap support, It's planned to embed a smal pop server in Jsmtpd. As more and more people would also need a imap support, it will be done in the future.

I've created a ticket in the project manager for maildir support, and modified the one for a pop/imap. You can check it at http://www.taldius.net/projects/jsmtpd.

Thanks for your interest and your feedback ;)