diff --git a/lib/email/sender.rb b/lib/email/sender.rb
index 3ca23cc6ff4..43f473889b5 100644
--- a/lib/email/sender.rb
+++ b/lib/email/sender.rb
@@ -435,12 +435,18 @@ module Email
     # Due to mail gem magic, @message.text_part and @message.html_part still
     # refer to the same objects.
     #
+    # Most imporantly, we need to specify the boundary for the multipart/mixed
+    # part of the email, otherwise we can end up with an email that appears to
+    # be empty with the entire body attached as a single attachment, and some
+    # mail parsers consider the entire email as a preamble/epilogue.
+    #
+    # c.f. https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
     def fix_parts_after_attachments!
       has_attachments = @message.attachments.present?
       has_alternative_renderings = @message.html_part.present? && @message.text_part.present?
 
       if has_attachments && has_alternative_renderings
-        @message.content_type = "multipart/mixed"
+        @message.content_type = "multipart/mixed; boundary=\"#{@message.body.boundary}\""
 
         html_part = @message.html_part
         @message.html_part = nil
diff --git a/spec/lib/email/sender_spec.rb b/spec/lib/email/sender_spec.rb
index 211da031705..830865a068c 100644
--- a/spec/lib/email/sender_spec.rb
+++ b/spec/lib/email/sender_spec.rb
@@ -681,6 +681,9 @@ RSpec.describe Email::Sender do
 
           Email::Sender.new(summary, "digest").send
 
+          expect(summary.content_type).to eq(
+            "multipart/mixed; boundary=\"#{summary.body.boundary}\"",
+          )
           expect(summary.attachments.map(&:filename)).to include(
             *[@secure_image, @secure_image_2, @secure_image_3].map(&:original_filename),
           )