I don't understand. You mean something Stalwart mail server on a raspberry pi or so?
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Basically yes, the service is needed to provide ingress and egress u/ElevenNotes has pointed out. I am inspired by a similar project called The Helm which sadly shut down last year. The hardware they were providing was much more powerful. I think they just tried to tackle too much and covid surely didn't help.
I would be more interested in an smtp relay service like ghettosmtp which could tunnel through CGNAT.