Article from SendGrid, our send layer.
The first step in helping ensure email deliverability is reputation. In the world of email, sending reputation refers to a set of specific metrics directly related to your email sending practices. Senders with good reputations get delivered. Senders with poor reputations get blocked at the gateway or their messages land in the "junk" folder instead of the inbox.
A strong sending reputation, like a great brand or personal reputation, is built over time. Here are the metrics ISPs look for when determining sending reputation:
Relevant, Properly Formatted Email
Sending quality emails that your subscribers want to receive is the basis of a great sending (and brand) reputation. Ensure that your recipients want to receive your email by implementing a clear opt-in during the subscription process and be sure to send relevant and interesting content. Also, make sure your HTML is properly formatted—poorly coded emails get caught in filters or don’t render properly.
Engaging Users and Cutting Disengaged Users
As a marketer you have access to data on users' deliveries, opens, and email clicks. Mailbox Providers have access to all this and more and grade the quality of a sender based on users' interaction with that mail. As a general rule, the higher your average engagement, the greater the chance that your messages land in the primary inbox. The inverse is also true: sending to users who do not engage results in a poor grade, and increases the chance of being sent to the spam folder.
How much email do you send? High-volume senders are always a red flag, especially when volumes are inconsistent. Do you send approximately the same number of emails each week or month, or is your sending schedule all over the map? Consistent volumes based on subscriber preferences are a key consideration for ISPs.
Very Few Complaints
Do your subscribers complain or tag your messages as "junk" or "spam"? Even a tiny increase in complaints can cause your email to be blocked by the ISPs. Keeping your complaint rate very low (less than .1% of email that is sent and accepted by the ISP) is very important.
Avoid Spam Traps
Sending to even one spam trap or “honey pot” will instantly set back your reputation and cause deliverability problems. When you send to a spam trap (an email address activated by an ISP to catch spammers), it means you’re engaging in email address harvesting (an illegal practice) or your list hygiene practices are weak. Either way, ISPs aren’t going to deliver your email.
Low Bounce Rates
A good reputation also means that only a small percentage of your emails "bounce" back or are returned by the ISPs because the account is no longer active (hard bounce) or the mailbox is temporarily full or the recipient is out-of-office (soft bounce). If a lot of your mail is bouncing back, it means your subscribers aren’t engaged and you’re not keeping up to date with them. It also indicates that your list hygiene practices are not up to industry standards. This makes your email look like spam to an ISP and your email is unlikely to get delivered. Keeping your bounce rate low by implementing procedures to immediately remove email addresses that return "hard" bounces is essential.
No Blacklist Appearances
Appearing on just one of the leading blacklists is enough to get you blocked by some ISPs. Senders with low complaints, who don’t hit spam traps, and who send email consistently generally don’t get blacklisted. However, if you do get blacklisted, having a good sending reputation will help convince the blacklist administrator to remove your IPs from their list.