Comment (by Roland Haas):

@Steven R. Brandt this is marked as “major” ie release relevant and needs to be fixed before the release. If indeed major then a solution should be provided (eg using Debian’s default Exim mail server, which will be happy to help set up since I use it myself) or downgrade this ticket.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2298/the-etk-website-does-not-build-with-the