Smtpscan is a tool to guess which MTA is used by sending several “special” SMTP requests and by comparing error codes returned with those in the fingerprint database. It does not take into account banners and other text information, that cannot be trusted, only error codes. A document describing the fingerprinting method implemented in smtpscan is available here.
You can download it from the following link: https://packetstormsecurity.com/files/download/31102/smtpscan-0.5.tar.gz
Source: https://packetstormsecurity.com/files/31102/smtpscan-0.5.tar.gz.html