On Mon, 9 Apr 2018, j.emerlik wrote: > SMTP lets you know the user agent, I thought that pop3 also. SMTP does not, at least, it's not part of the protocol. The User-Agent is a header optinally inserted by mail clients within the message. IMAP ID is an extension of the IMAP protocol https://tools.ietf.org/html/rfc2971 Joseph Tam <jtam.home at gmail.com>