These days people have set up a raspberry pi as an email server.
The Gmail app on android is store and forward when connected ie works off line.
If serval could be used as the transport layer and a raspberry pi hung off a mesh extender then LAN based email should be possible. LAN based would eliminate the possibility of spam.
Taking this further email messages to go off the LAN could be sent to a designated android device, forwarded to an email address on the internet then sent on. The crude way would be carry it to an internet cafe, send and receive emails.
Return messages could be handled manually the same way. ie please send xyz to the field etc.
Lots of practical considerations like packaging the raspberry pi or similar, perconfiguring the email server and looking after the accounts on the LAN.
Would you want to use Gmail or another app? etc etc.
Cheerio John