The mail server denied access to the account "..." because an administrator or other mail client was using it when Mail tried to log in. Try again later.

The server returned the error: The attempt to send data to the server “(null)” failed.

Eventually, Mail gets my mail, but sometimes only after a delay of as much as an hour.  I encounter this problem many times a week, sometimes several times in a single day.  On some days, though, the problem does not appear at all.

Any ideas on why this is happening and/or what might be done to fix/avoid it?