I use wordpress post via email feature, and use default script, wp-mail.php which connects to emailbox, reads emails and posts them into blog.
Almost everything was working OK, untill I noticed lots of dublicated posts.

I tried to run the file directly, and almost all posts were ok, but script stucked on one post(email message).  I reloaded the script(wp-mail.php). The results was the same. Script read all the messages AGAIN, and stuck on the same post again, with the error:  Oops POP3 delete: Command failed . Reloaded again. The same result again. This way I inserted triple posts And mailbox was not cleaned, the inserted messages were not deleted.

I’ve made a simple solution. WP-mail.php closes POP3 session and quits:

if(!$pop3->delete($i)) {
echo ‘<p>’ . sprintf(__(‘Oops: %s’), esc_html($pop3->ERROR)) . ‘</p>’;
$pop3->reset();
exit;
}

POP3 reset command resets any messages which have been marked as read or deleted to the standard unread state. So they will be posted again on the next POP3 session. I decided to make it easy way. Instead of reset messages, I forced quit command:

if(!$pop3->delete($i)) {
echo ‘<p>’ . sprintf(__(‘Oops: %s’), esc_html($pop3->ERROR)) . ‘</p>’;
$pop3->quit(); /* forced quit without reset */
exit;
}

As far as all read messages on session quit are deleted, I got rid off that stuck errors. And that error post is NOT inserted into my blog.

I hope this may help save some time to other bloggers. If this file is overwritten when updating wordpress, you need to modify wp-mail.php again.

This solution may be valid only if you have many messages, and only sometimes this error occurs. If you get this error on ALL messages, I suppose this solution it NOT for your case, as there are some other errors, like possible message formatting, character encoding errors.