Use deliver_method :test for in test environment - thanks to @corny.

app/models/channel/driver/sendmail.rb

@@ -7,7 +7,14 @@ class Channel::Driver::Sendmail
     return if Setting.get('import_mode')
 
     mail = Channel::EmailBuild.build(attr, notification)
-    mail.delivery_method :sendmail
+    mail.delivery_method delivery_method
     mail.deliver
   end
+
+  private
+
+  def delivery_method
+    return :test if Rails.env.test?
+    :sendmail
+  end
 end