Stamper will timestamp a whole received email message sent to stamp-eml@stamper.itconsult.co.uk, which will return a message to the sender containing both a detached PGP signature, and the original message as an EML attachment.
Once the attachments have been saved, this can be validated:-
$ gpg --verify 2114744.asc 2114744.eml gpg: Signature made Sun 21 Dec 2025 14:32:35 UTC gpg: using EDDSA key FF20737BA3781A1D9FBB8CB1E8E883D10F6F2CC2 gpg: Good signature from "stamper.itconsult.co.uk - Timestamping Key" [full]