Best way to test high-volume SMTP email sending code?

Question

I\'ve written a component in a Windows service (C#) which is responsible for sending sometimes large volumes of emails. These emails will go to recipients on many domains &ndas

Answer 1

After searching around, I ended up firing up Papercut on several extra machines I had laying around. I then populated my database with test addresses *@[test-machine-*.local].

While this did work well enough, I tested with 25 send threads and it looked like I was overwhelming the four computers running Papercut. Several hundred send attempts experienced TCP connection failures; those messages were properly requeued to be sent later (and ultimately did arrive). However, out of 25,000 test emails, about 500 simply disappeared – adding up the *.eml files in Papercut's folder on each test machine yielded only ~24,500.

Now I'm left wondering whether the missing emails are a due to a problem in my code, or if Papercut dropped messages which it reported in SMTP as 250 OK.