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questo.email was an email-to-webmention bridge that was live from December 2014 through December 2015.  The code is available at: https://github.com/fiatjaf/questo.email.
How to use
If you're using it from the indieweb side (your own site), it supports mentioning and messaging people by email, just like a webmention to the home page, but to an email instead (you just have to send the webmention to
If you're using it from the email side (your own email messages), it does webmentions in reply to posts (at
email@example.com and the URL in the Subject) and webmentions to the home page (messaging) (at
firstname.lastname@example.org). It also supports basic publishing at
email@example.com (like a webmention, but not targeted at anyone, yet content will be live on the web -- this kind of post also sends webmentions to any referred URL).
More detailed instructions are found at questo.email's home page.
Gregorlove.com has risen the concern that questo.email may be a easy source of spam attacks, fiatjaf.alhur.es has drafted some of the initial questo.email anti-spam features in response.
fiatjaf.alhur.es has a questo.email page (from his primary email) at fiatjaf.gmail.com.questo.email and is building a full integration for his site, planning to slowly migrate all his email interactions to the web.