Cannot move mail to mailbox on my computer

Question

Suddenly my Mail program will not move messages from the institutional server I use to my own "On My Mac" folder. The message I receive is "304 messages could not be moved to the mailbox “Incoming mail 2006 — On My Mac”. This does not vary no matter the number of messages, or to which mailbox on my computer, old or newly created.
I cannot find any answer to this in "Mail Help." Any suggestions welcome.

Answer 1

Verify/repair the startup disk (not just permissions), as described here:
The Repair functions of Disk Utility: what's it all about?
After having fixed all filesystem issues, if any, and making sure that there’s enough space available on the startup disk (a few GB, plus the space needed to make a backup copy of the Mail folder), try this:
1. Quit Mail if it’s running.
2. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong, e.g. by dragging it to the Desktop while holding the Option (Alt) key down. This is where all your mail is stored.
3. Locate Envelope Index and move it to the Trash. If you see an Envelope Index-journal file there, delete it as well.
4. Move the “IMAP-” account folders to the Trash. Note that you can do this with IMAP-type accounts because they store mail on the server and Mail can easily re-create them. DON’T trash any “POP-” account folders, as that would cause all mail stored there to be lost.
5. Open Mail. It will tell you that your mail needs to be “imported”. Click Continue and Mail will proceed to re-create Envelope Index -- Mail says it’s “importing”, but it just re-creates the index if the mailboxes are already in Mail 2.x format.
6. As a side effect of having removed the IMAP account folders, those accounts may be in an “offline” state now. Do Mailbox > Go Online to bring them back online.
Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder, i.e. ~/Library is the Library folder within the user’s home folder.