Progress: C SMTP proxy reliably logs all I/O, dies when client or server dies, does not interfere at all (yet!). https://gist.github.com/schmonz/6d8da539a5a43e99d5ad5810cde05b22#file-qmail-fixsmtpio-c
Next: parse requests a little, pick an SMTP verb to intercept, respond directly.
Nextnext: intercept the verb iff told to by a config file.