diff --git a/changelog.d/hackney-castore.security b/changelog.d/hackney-castore.security
index 44cb90ce6..8d1a0cf4a 100644
--- a/changelog.d/hackney-castore.security
+++ b/changelog.d/hackney-castore.security
@@ -1 +1 @@
-Hackney and Finch now trust only the CAStore certificate bundle, which rejects the compromised eMudhra intermediate CAs
+Hackney, Finch, Mua and the Swoosh HTTP mail adapters now trust only the CAStore certificate bundle, which rejects the compromised eMudhra intermediate CAs
diff --git a/lib/pleroma/emails/mailer.ex b/lib/pleroma/emails/mailer.ex
index fa6fd8020..e6abf4456 100644
--- a/lib/pleroma/emails/mailer.ex
+++ b/lib/pleroma/emails/mailer.ex
@@ -68,6 +68,21 @@ defmodule Pleroma.Emails.Mailer do
   end
 
   defp parse_config(config) do
-    Swoosh.Mailer.parse_config(@otp_app, __MODULE__, @mailer_config, config)
+    @otp_app
+    |> Swoosh.Mailer.parse_config(__MODULE__, @mailer_config, config)
+    |> put_mua_cacertfile()
+  end
+
+  @doc false
+  # Mua otherwise trusts the OS CA store. Keep a CA that the admin set.
+  def put_mua_cacertfile(config) do
+    ssl = Keyword.get(config, :ssl, [])
+
+    if config[:adapter] == Swoosh.Adapters.Mua and
+         not Keyword.has_key?(ssl, :cacertfile) and not Keyword.has_key?(ssl, :cacerts) do
+      Keyword.put(config, :ssl, [{:cacertfile, CAStore.file_path()} | ssl])
+    else
+      config
+    end
   end
 end
diff --git a/test/pleroma/emails/mailer_test.exs b/test/pleroma/emails/mailer_test.exs
index 2c2b5a4af..d42e8b723 100644
--- a/test/pleroma/emails/mailer_test.exs
+++ b/test/pleroma/emails/mailer_test.exs
@@ -61,4 +61,33 @@ defmodule Pleroma.Emails.MailerTest do
     assert hackney_options[:ssl_options] ==
              Pleroma.HTTP.TLS.ssl_options()
   end
+
+  describe "put_mua_cacertfile/1" do
+    test "makes Mua trust the CAStore bundle" do
+      config = Mailer.put_mua_cacertfile(adapter: Swoosh.Adapters.Mua, relay: "mail.example.com")
+
+      assert config[:ssl] == [cacertfile: CAStore.file_path()]
+      assert config[:relay] == "mail.example.com"
+    end
+
+    test "keeps other ssl options" do
+      config = Mailer.put_mua_cacertfile(adapter: Swoosh.Adapters.Mua, ssl: [depth: 3])
+
+      assert config[:ssl] == [cacertfile: CAStore.file_path(), depth: 3]
+    end
+
+    test "keeps a CA that the admin set" do
+      config = [adapter: Swoosh.Adapters.Mua, ssl: [cacertfile: "/etc/ssl/smtp.pem"]]
+      assert Mailer.put_mua_cacertfile(config) == config
+
+      config = [adapter: Swoosh.Adapters.Mua, ssl: [cacerts: []]]
+      assert Mailer.put_mua_cacertfile(config) == config
+    end
+
+    test "leaves other adapters alone" do
+      config = [adapter: Swoosh.Adapters.SMTP, relay: "mail.example.com"]
+
+      assert Mailer.put_mua_cacertfile(config) == config
+    end
+  end
 end