You can selfhost the email server wherever you want. But you’ve to use some external system to deliver the email or you’ll end up in spam because your residential IP is most likely dynamic and already flagged by most email providers.

One way to do it is to get a VPS somewhere and setup Wireguard on it. Then configure your local system to bind to the Wireguard interface and IP so all email send and received using the tunnel. Dovecot doesn’t care what interface it is running on, Postfix has specific options that you can change in master.cf to accommodate the fact that it will be binding to the VPN IP and the real IP is the VPS public IP.

1. Setup a install of Dovecot / Postfix / Rspamd on your local server: https://workaround.org/ispmail-bookworm/
2. Start by setting up a Wireguard tunnel between your local server and the VPS: https://www.digitalocean.com/community/tutorials/how-to-set-up-wireguard-on-ubuntu-20-04
3. Create a outgoing transport for the email that uses the WG tunnel and is aware of the VPS public IP:

out-wg      unix  -       -       n       -       -       smtp
 -o proxy_interfaces=188.xxx.xxx.xxx # the real public IP of the VPS
 -o smtp_bind_address=10.0.0.2 # the IP that your local server has on the WG interface
 -o inet_interfaces=10.0.0.2 # same as above
 -o myhostname=server.example.org # should match the PTR / reverse DNS entry on the VPS IP
 -o smtp_helo_name=server.example.org # should match the PTR / reverse DNS entry on the VPS IP
 -o syslog_name=smtp-wg

4. Set your VPS firewall to NAT/forward incoming traffic on port 25, 587, 465 and 993 to the local server (wireguard client 10.0.0.2);
5. Change main.cf to use the transport by adding: default_transport = out-wg.

That’s everything you need to get it going. Use https://www.mail-tester.com/ to debug if DKIM and everything else is properly setup at the end.