Suggestion: OpenTimeStamping should be also working via mail (receive a mail, timestamp/sign it, and reply to sender)

[Wikinaut]

See http://www.itconsult.co.uk/stamper.htm for an implementation started back in 1995 which uses PGP.

[petertodd]

I'm quite familiar with that service actually; it's unfortunate that he hasn't updated his key to fix the MD5 hash and missing self-signatures. That said it's remarkable how long the service has been in operation.

OpenTimestamps can certainely support such features in the future and with some work could probably even represent a timestamp made by his service.