I'm getting this message whenever I try to send an email or reply to an email from a POP email address.
Also, I want to note, that I do have other email addresses that are able to send and reply with no issue, so I assume it has something to do with a particular account or outgoing mail server.
I have been in contact w/ a remote user who has this same issue.
He has a rather large volume of local mail in a POP account. He recently had to migrate his data from one Macbook Pro running Snow Leopard to another so that we could send his older machine in to get repaired.
Now that he's working on the loaner with all his data migrated over, he gets this error when he tries to send mail. He cannot send any mail from within Mail.app. I'm currently having him back up his ~/Library/Mail/ and ~/Library/Mail Downloads/ and ~/Library/Preferences/com.apple.mail.plist files to an external drive before we really start messing about with anything too scary. I had him try to remove the envelope index file to "import" all the mail, but mail crashed twice at about 30% complete, so he put the old envelope index file back.
Something interesting: he has two accounts, The one which does not work is the work account which is set up for POP3 and one is a personal account which is set up using IMAP.
He just sent me a test message from the personal account and mail had no problem writing to that account's (do they have separate outboxes?) outbox.
He does have like 400,000 messages total in a variety of folders in mail, but... it was working before he used the Migration Assistant to move his data to this temporary laptop.
There is only one Outbox if all is properly set up. However, despite the one account being able to send, I would quit Mail, and in the Finder open Home/Library/Mail/Mailboxes and locate the Outbox.mbox folder -- is there? If so, delete it, relaunch Mail and immediately try to send from the account that has not been working. Mail should create a new Outbox.mbox folder, and then send.
