If your lan devices only have a fe80, your clients are not receiving a proper router advertisement.
Which routes and firewall rules should I be checking?
Since the OPNsense device is getting a ipv6 address and is able to ping ipv6 devices on the internet.
It sounds like you don’t have ipv6 configured for the LAN. Try enabling “Assisted” mode.
Avoiding spam filters and ISP blocks are a common issue with self hosted email.
Ideally you’ll have a mature domain, dkim configured and an IP that’s not blacklisted on a network that allows email traffic.
OVH offers pretty cheap email hosting if you just want a turn key solution.
Otherwise if you want your home lab to work this way you may need to configure something akin to a forward proxy on a VPS to act as a gateway for your homelab.
This could be achieved using wireguard and iptables. By routing the email traffic to your homelab.