

How do you expect the packets to actually route? If you run Tailscale and your VPN on your phone, they might fight with each other for control of the routing table.
If you’re trying to use Tailscale exit note to then route through Tailscale to one node running gluetun to Mullvad. That’s going to be complex because against they both want to mess with the routing table.
Tailscale natively supports Mullvad: https://tailscale.com/mullvad
I’ve been running my own mail for 10+ years. I recommend rspamd for spam filtering. It took the place of SpamAssasin, grey listing, SPF checking, etc. All in one single system.