If you see the following errors in your /var/log/maillog, it likely means that your /etc/virtual/domain.com/passwd file has not been updated to the dovecot format. The passwd file, with dovecot, requires 7 columns, colon separated. If you only see 2, then your passwd file hasn't been converted.
To resolve this issue, first ensure that you have:
present in your /usr/local/directadmin/conf/directadmin.conf.
Next, run the following:
(If you're using freebsd, the directadmin restart is: