Look into WInlink. You have to have an amateur radio license, and you’ll have to choose between different software (ie: radio modem), and you’ll need an HF transceiver. Once licensed you can create a Winlink account (via radio, no web signups!), which gives you access to smtp through their gateways (which are actually owned by other hobbyists). Different towers/repeaters have different specs/protocols (PACTOR, Packet, I use VARA) but all of them allow you to send email to any domain or recipient, just like regular email. Hardest part is configuring your laptop (or even phone) to control your transceiver to send the signals out. The email you’d have is your_callsign@winlink.org and with HF frequencies you can hit just about any gateway listed here https://winlink.org/content/gateway_locations