We had an issue where my mom's company had email set up through like, godaddy or some shit. And to have calendar invites work (including people sending google calendar invites, or my mom sending them invites), it was some extra charge that her boss wasn't willing to pay. I explored a bunch of possibilities, then on a whim threw Thunderbird on her machine, set up IMAP/SMTP, and tried sending her a calendar invite. It Just Worked, showed up in her Thunderbird calendar. Then she did the same for me, boom, functional calendar invite.

All absolutely free of charge.

I think that's a good thing, that's what Mozilla is good at doing, subverting arbitrary cost added bullshit.