Catalina Mail Server

Hi

I'm setting up a mail server on MacOS Catalina using Server and and the third-party package Server Plus. Server Plus uses PAM and OpenDirectory authorisation in Dovecot and this works fine for Local Network Users but not for local users such as the Administrator account. I've checked the way Apple has done authorisation in Server 5 and this uses 10-auth-submit.conf.ext as well as OpenDirectory and in particular the submit-passdb file. I haven't found a way of creating the submit-passwd file so perhaps there's another way to do things. Has anyone set up a mail server on Catalina using Open Directory and achieved authorisation by local as well as network users?