Comment (by Roland Haas):

Please have a look at the branch rhaas/debian10 which sets up and uses exim. Works to send email if I start the docker container on my workstation and replace the smarthost setting so that it matches NCSA’s smarthost.

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