Email deliverability can be affected by all of these factors:
a) Reputation of sending domain
- Sending emails to bad addresses repeatedly - When the emailer sends repeatedly to bad addresses, ISP’s record this and subsequently block the sender. It looks like the sender is trying a dictionary attack. You must clean your email list, taking out email addresses that bounce.
- Sending emails to spamtraps. These are traps set up by blacklists to catch spammers. The addresses are never given out, so sending to them will immediately land you on a blacklist.
- How quickly/slowly emails are sent (throttling). ISP’s frown upon messages sent in large bunches. It’s also better to have spam complaints trickle in rather than come in large bunches.
- Users hitting the spam button. ISP’s take this very seriously. You can usually sign up for a feedback loop that will send you this information so that you can clean those addresses off your list. Failure to do so will rapidly deteriorate your deliverability.
- Sending architecture (SPF records, DKIM, etc.) that identifies sender properly – without this there is no reputation, and no reputation=bad reputation.
- Challenge-response system responses: the ISP sends a response requiring the sender to reply before the email will be sent on to the recipient.
- Communication between servers (Brightmail). Brightmail servers share info about spammers.
b) Content of the email message
- Criteria varies by filter (SpamAssasin is one example), includes things like verbage in subject line, excessive graphics or scripting, large bright colored fonts, certain keywords, cleanliness of code, etc.
c) Blacklists
- Blacklists are lists kept by independent organizations or internally by ISP, that record IP/domains of known spammers. ISP’s often consult the blacklists of independent organizations. There are many of them. Whitelists are the antithesis of blacklists. There are whitelists managed by independent organizations or managed internally by ISP.
Most important of all these, is the reputation of the sending domain. It used to be that ISP's filtered based on the content of the email. But more and more, ISP's are focusing on the reputation of the sending domain (or IP). Given the complexity involved, and the fact that reputation is the most important factor, if you do any sort of decent volume of outbound mail, you probably SHOULD be outsourcing your email delivery.