If you are suddenly getting "LMTP error after RCPT TO" error message when users send emails to your forwarded email address, then verify that the forwarded email address was not created at some point in time and then deleted as POP3/IMAP box.

Do that by deleting the entry at /home/$username/mail/$domainname.com/Usermailbox

For example, if the issue was with support@somedomain.com , then delete the entry:

/home/$username/mail/somedomain.com/support

Only do this if you're using forwarder only, and there is no mailbox exist for that user.