You can do the same with swoosh.
An example firezone/mailer_test.exs at 4b67809c9bfc87d54001332904a179e8be804c4a · firezone/firezone · GitHub
And corresponding HTML & text templates firezone/apps/fz_http/test/fz_http/mailer_test/sample_email at 4b67809c9bfc87d54001332904a179e8be804c4a · firezone/firezone · GitHub
It uses Phoenix.View so it does the same thing as Phoenix. Using an atom means rendering all possible formats.