This message appears when I open Mac Mail:
The mailbox [quote]~/Library/Mail/Mailboxes/Outbox.mbox {quote] is currently locked by [quote](null)[quote] on host [quote][quote]. If you open a mailbox that already is in use, you may damage its content
I found a similar message on a forum, but the suggested solution did not work because my >home folder>library>mail folder did not contain a folder about envelope.
I have an iMac, using OS 10.3