Is it possible with Postfix to add a warning to all emails received over a cleartext (non-TLS) connection?
For example modifying the subject and/or adding a custom header.
The way to go here is to first tell Postfix to add the TLS connection status into the Received header. In
smtpd_tls_received_header = yes
This will add something like
(using TLSv1.2 with cipher … (256/256bits)) to the postfix generated received header.
This information can then be used to add a custom headers. E.g. one could add a
X-Transport-Layer-Security header and set its value depending on whether the above snippet is present in the Received header. Possible methods to do so include
I don't know of any ready-made solution to do this, but it shouldn't be much work either.