Why is my opendmarc failing pretty much everything that comes through?

I also had this issue recently. In my case I managed to resolve it by adding the following to /etc/opendmarc.conf:

IgnoreAuthenticatedClients true

man opendmarc.conf has this to say about it:

IgnoreAuthenticatedClients (Boolean)
   If set, causes mail from authenticated clients (i.e., those that used SMTP AUTH) to be
   ignored by the filter.  The  default  is "false".

which is exactly what I wanted. I only allow external connections to the SMTP via secure connections. Now opendmarc leaves my outgoing email alone.

Try change main.cf

smtpd_milters =     inet:mopsmailer_spamassassin:784 inet:localhost:8891 inet:localhost:8893
non_smtpd_milters = inet:mopsmailer_spamassassin:784 inet:localhost:8891 inet:localhost:8893

OpenDKIM check first! Next is OpenDMARC check...