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.
This can be validated:-
[top be updated once upgrades have gone live] $ gpg --verify 3000049.asc 3000049.eml gpg: Signature made Sat 11 Oct 2025 11:06:13 UTC gpg: using EDDSA key C3172BA102CD3FA8BC2C20719699A1E8328D65AD gpg: Good signature from "TEST stamper.itconsult.co.uk - Timestamping Key" [full]