How hard is it to host your own email on a nextbox with security stability and good spam policy? Also a xmpp server is an interesting idea to have your own IM for family and friends.
the answer strongly depends on how comfortable you are with configuring those on a Debian system. With the NextBox you essentially get a Raspberry PI4 with full access via ssh. Thus the answer is maybe also: similar complex as on any other Debian RaspberryPI4 system.
A important hint might be: you should avoid setting up a mail-server on a dynamic IP address (as most consumer ISP connections are) because with a regular changing IP your mail server will likely be flagged as spam server.