Today when sending an email campaign out the most important factor is your IP and domain reputation. In the old days, content filtering was the main method of filtering email into whether it was accepted and or if ends up in the bulk/junk/spam folder.
Now everything is based on user engagement. Meaning any action that the user does to an email message, whether it is opening the message, clicking links in the message, replying to the message, marking the email as spam, deleting the message, adding the contact to your address book, etc.
Tips for Warming up IPs and Domains.