I used a proxy like this for a few years, though in my case it was to handle spam. I based my work on this extensible / plugin-based perl daemon: http://smtpd

I used a proxy like this for a few years, though in my case it was to handle spam. I based my work on this extensible / plugin-based perl daemon: http://smtpd.github.io/qpsmtpd/ http://smtpd.github.io/qpsmtpd/ I'd reject spam at SMTP time, but also capture the rejected messages, so that users could view them in an online quarantine. Worked pretty well, though I think these days the original perl-based project has been reworked into a nodejs thing: https://haraka.github.io/about/ https://haraka.github.io/about/