1. Ok, so download Hex Fiend from the link shown by other users.
Also stop mBox server, your email client, etc.
2. Go to the /usr/local/FluentFactory/mBoxMail folder with Finder (use Go/Go To Folder menu to get into the hidden /usr folder)
3. Make a copy of the mBoxMail file
4. Open file mBoxMail with Hex Fiend using Open With...
5. In Hex Fiend, choose Edit/Find/Find...
6. In the panel that showed up on top, choose the ASCII radio button
7. Enter AUTH=PLAIN in the Find field
8. Enter 10 spaces in the Replace field
9. Execute "Replace All"
10 Try File/Save
11. It may not work because of rights issues. Dismiss the error message and leave the Hex Fiend running.
12. Separately open a Terminal window
13. Execute "cd /usr/local/FluentFactory/mBoxMail"
14. Execute "whoami". The name that shows up is your account alias. Mine is resell, yours is different. Use yours in the next command
15. Execute "sudo chown reseul mBoxMail". You may get prompted for password. Enter it.
16 Back to Hex Fiend, try File Save again. It should work now.
17 Back to terminal, execute "sudo chown root mBoxMail" to restore the right owner (root).
You're done. Close the hex editor, terminal, restart the mbox server..
Now, this may not be the most optimal path. I tried my best.
I hope it helps.