It worked, perhaps I will file a bug report for :pleromatan2:
I'm not certain why those dependancies were needed but they weren't in the mix.exs perhaps they can be added.
For reference, if you are using mailgun for your transactional email with pleroma you'll need plug and multipart deps added to your mix.exs
You add them to this portion of the file, I just picked a random version of each dependancy
defp deps do[
{:plug, "~> 1.12"}, # Plug library for working with connections and query strings
{:multipart, "~> 0.4.0"} # Multipart library for handling multipart form data
]
end