What do you all use / recommend for mailing list software?

I help a local charity with their website, and we have a number of Google Groups (with our domain) for our internal communication (no external marketing). However, the email will frequently be classed as spam by Yahoo and Outlook. I've been in touch with their postmaster teams, and each time they will first blame Google and then tweak their spam filters, which works for a week and then our list email goes to spam again.

I wouldn't be surprised to learn that it being filtered as spam due to the Google Group footer - which can't be disabled for external email addresses.

I'm fed up with the whack-a-mole, and wondering what alternatives there are? I just need a central email address that members can email, and that then be sent out to the members.