I pre-test my emails by sending them to: https://www.mail-tester.com/
I create a Smart Campaign to [Send Alert] to Other Emails (specify the email you want to test and copy/paste the email address that mail-tester provides). Then, check your score on mail-tester and make the improvements it suggests.
We use Litmus, From what I understand, 250ok uses Litmus data and expands upon it. Litmus does a great job for us, it explains SPAM Problems by ESP and provides a ton of views across many ESP and device views.