Some mail clients put a User-Agent string in the headers. E.g. User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 User-Agent: Alpine 2.20 (LFD 67 2015-01-07) There's an Add-On for Thunderbird which will (usually) tell you what mail client the send used. https://addons.mozilla.org/en-GB/thunderbird/addon/display-mail-user-agent/