outlook office setup encryption on Mac

We are leaving another hipaa encrypted email service and attempting to make outlook work for our mental health clinic. The goal is to be able to encrypt and unencyrpt emails on our own. However we would like the default to always be encrypted. So my fear is that I can't relay on my staff to remember to click encrypt all the time. My hope is that we can default the encryption and then select unencrypt when we want the emails to be sent to list serves for example.

I have tried to work with outlook and we tried to make a "rule " but in order to make that work everyone would have to send me their emails they want un-encypted in advance and I would have to make them a exception. We tried making a "label" but that backfired. and the last place we left off was trying to make a s/mime but Mac wouldn't make me a certificate and I realized I am over my head with this.