diff --git a/CHANGELOG.rdoc b/CHANGELOG.rdoc
index 4727518dd..4eae71cdf 100644
--- a/CHANGELOG.rdoc
+++ b/CHANGELOG.rdoc
@@ -11,6 +11,7 @@ Features:
 
 * Updated README to improve around sending multipart mail @kapfenho
 * Add delivery_interceptors method to Mail class to fetch registered interceptors @ghousemohamed
+* `Mail::ContentTypeField` now supports setting a `boundary_generator`, allowing custom boundary creation behavior @dugancathal
 
 Code Improvements:
 
@@ -19,6 +20,7 @@ Code Improvements:
 * Configure RSpec's zero-monkey patching mode @olleolleolle
 * Added support for JRuby 9.4 @mikel
 * Prefer `__dir__` @olleolleolle
+* Remove `=` from default mime boundary in `Mail::ContentTypeField` @dugancathal
 
 Bug Fixes:
 
diff --git a/lib/mail/fields/content_type_field.rb b/lib/mail/fields/content_type_field.rb
index 686d533ae..8ccda08a5 100644
--- a/lib/mail/fields/content_type_field.rb
+++ b/lib/mail/fields/content_type_field.rb
@@ -14,10 +14,13 @@ def with_boundary(type)
         new "#{type}; boundary=#{generate_boundary}"
       end
 
+      attr_accessor :boundary_generator
+
       def generate_boundary
-        "--==_mimepart_#{Mail.random_tag}"
+        "--#{boundary_generator.call}"
       end
     end
+    self.boundary_generator = -> { "_mimepart_#{Mail.random_tag}" }
 
     def initialize(value = nil, charset = nil)
       if value.is_a? Array
diff --git a/spec/mail/fields/content_type_field_spec.rb b/spec/mail/fields/content_type_field_spec.rb
index 5753faf17..87175886a 100644
--- a/spec/mail/fields/content_type_field_spec.rb
+++ b/spec/mail/fields/content_type_field_spec.rb
@@ -180,14 +180,21 @@
   end
 
   describe "class methods" do
+    def with_generator(generator)
+      old, Mail::ContentTypeField.boundary_generator = Mail::ContentTypeField.boundary_generator, generator
+      yield
+    ensure
+      Mail::ContentTypeField.boundary_generator = old
+    end
+
     it "should give back an initialized instance with a unique boundary" do
       boundary = Mail::ContentTypeField.with_boundary('multipart/mixed')
-      expect(boundary.encoded).to match(%r{Content-Type: multipart/mixed;\r\n\sboundary="--==_mimepart_[\w]+_[\w]+"\r\n})
+      expect(boundary.encoded).to match(%r{Content-Type: multipart/mixed;\r\n\sboundary=--_mimepart_[\w]+_[\w]+\r\n})
     end
 
     it "should give back an initialized instance with different type with a unique boundary" do
       boundary = Mail::ContentTypeField.with_boundary('multipart/alternative')
-      expect(boundary.encoded).to match(%r{Content-Type: multipart/alternative;\r\n\sboundary="--==_mimepart_[\w]+_[\w]+"\r\n})
+      expect(boundary.encoded).to match(%r{Content-Type: multipart/alternative;\r\n\sboundary=--_mimepart_[\w]+_[\w]+\r\n})
     end
 
     it "should give unique boundaries" do
@@ -198,6 +205,12 @@
       end
     end
 
+    it "should allow configuring a boundary generator" do
+      with_generator(-> { 'arbitrary-boundary' }) do
+        boundary = Mail::ContentTypeField.with_boundary('multipart/alternative')
+        expect(boundary.encoded).to match(%r{Content-Type: multipart/alternative;\r\n\sboundary=--arbitrary-boundary})
+      end
+    end
   end
 
   describe "Testing a bunch of email Content-Type fields" do
diff --git a/spec/mail/mime_messages_spec.rb b/spec/mail/mime_messages_spec.rb
index 024b68684..c6d41b708 100644
--- a/spec/mail/mime_messages_spec.rb
+++ b/spec/mail/mime_messages_spec.rb
@@ -282,14 +282,14 @@
           content_type "text/html; charset=US-ASCII"
           body "<b>This is HTML</b>"
         end
-        expect(mail.to_s).to match(%r|Content-Type: multipart/alternative;\s+boundary="#{mail.boundary}"|)
+        expect(mail.to_s).to match(%r|Content-Type: multipart/alternative;\s+boundary=#{mail.boundary}|)
       end
 
       it "should set the content type to multipart/alternative if you declare html and text parts" do
         mail = Mail.new
         mail.text_part { }
         mail.html_part { }
-        expect(mail.to_s).to match(%r|Content-Type: multipart/alternative;\s+boundary="#{mail.boundary}"|)
+        expect(mail.to_s).to match(%r|Content-Type: multipart/alternative;\s+boundary=#{mail.boundary}|)
       end
 
       it "should not set the content type to multipart/alternative if you declare an html part but not a text part" do