Someone using a rust implementation of telnet listening to port 25 to answer incoming requests.

No experience but there seems to be one here: https://james.apache.org/

It has "enterprise" in the name though so it's unlikely to be very robust or secure.

Thunderbird, given that the mix of JavaScript + C++ should be less of a problem for buffer overflows.

Otherwise, business opportunity for other language communities.

There's maddy, but it's very new and not very popular.