Provide sane defaults for SMTP

OTP’s default SSL/TLS settings are rather restricitive
and in particular do not use system CA certs.
In our case using system CA certs is virtually always desired
and the lack of it leads to non-obvious errors. Manually configuring
system CA certs from in-database config also isn’t straightforward.

Furthermore, gen_smtp uses a different set of connection options
for direct SSL/TLS and a later TLS upgrade providing additional
confusion and complexity in how to configure this.

Thus provide some suitable defaults for sending SMTP emails.
Everything can still be overriden by admins if necessary.

Note: defaults are not appended when validating the config
in hopes of improving the error message (as the required relay key
is already accessed to generate defaults for optional fields)

Fixes: https://akkoma.dev/AkkomaGang/akkoma/issues/660

lib/pleroma/emails/mailer.ex

@@ -55,12 +55,61 @@ defmodule Pleroma.Emails.Mailer do
 
   @doc false
   def validate_dependency do
-    parse_config([])
+    parse_config([], defaults: false)
     |> Keyword.get(:adapter)
     |> Swoosh.Mailer.validate_dependency()
   end
 
-  defp parse_config(config) do
-    Swoosh.Mailer.parse_config(@otp_app, __MODULE__, @mailer_config, config)
+  defp ensure_charlist(input) do
+    case input do
+      i when is_binary(i) -> String.to_charlist(input)
+      i when is_list(i) -> i
+    end
+  end
+
+  defp default_config(adapter, conf, opts)
+
+  defp default_config(_, _, defaults: false) do
+    []
+  end
+
+  defp default_config(Swoosh.Adapters.SMTP, conf, _) do
+    # gen_smtp and Erlang's tls defaults are very barebones, if nothing is set.
+    # Add sane defaults for our usecase to make config less painful for admins
+    relay = ensure_charlist(Keyword.get(conf, :relay))
+    ssl_disabled = Keyword.get(conf, :ssl) === false
+    os_cacerts = :public_key.cacerts_get()
+
+    common_tls_opts = [
+      cacerts: os_cacerts,
+      versions: [:"tlsv1.2", :"tlsv1.3"],
+      verify: :verify_peer,
+      # some versions have supposedly issues verifying wildcard certs without this
+      server_name_indication: relay,
+      # the default of 10 is too restrictive
+      depth: 32
+    ]
+
+    [
+      auth: :always,
+      no_mx_lookups: false,
+      # Direct SSL/TLS
+      # (if ssl was explicitly disabled, we must not pass TLS options to the socket)
+      ssl: true,
+      sockopts: if(ssl_disabled, do: [], else: common_tls_opts),
+      # STARTTLS upgrade (can't be set to :always when already using direct TLS)
+      tls: :if_available,
+      tls_options: common_tls_opts
+    ]
+  end
+
+  defp default_config(_, _, _), do: []
+
+  defp parse_config(config, opts \\ []) do
+    conf = Swoosh.Mailer.parse_config(@otp_app, __MODULE__, @mailer_config, config)
+    adapter = Keyword.get(conf, :adapter)
+
+    default_config(adapter, conf, opts)
+    |> Keyword.merge(conf)
   end
 end