Hello, I've noticed on server same problem:
(temporary failure. Command output: /usr/bin/maildrop: Unable to create a dot-lock at /var/vmail/<DOMAIN.LTD>/<DOMAIN.LTD>/28144.0.<DOMAIN.LTD>.) username@<DOMAIN.LTD>
I've checked in /var/vmail and this folder/user didn't exist, but as suggested here:
i run the command:
# sendmail -v -q
and e-mail was sent/folder/user was created in /var/vmail.
Any idea what is the problem here ?