Foundations of Email Deliverability

Mail reaches the inbox when the people receiving it both want it and expect it. The two conditions are different, and each one counts:

1. Wanted: the message gives the recipient something they value.
2. Expected: the recipient recognizes the mail, because they signed up for it or already do business with your brand.

How mailbox providers evaluate your email

No mailbox provider or internet service provider (ISP) can ask each recipient whether a message was welcome. What a provider can do is watch how its users treat your mail across tens of thousands of mailboxes, and that behaviour is a more reliable answer than asking. Complaints and engagement weigh the most. For each campaign, a provider can see:

• The share of recipients who reported it as spam
• The share who deleted it unread
• The share who opened and read it
• How much time readers spent on it

Filtering algorithms combine these measurements into a judgement on whether recipients want and expect mail from you.

Content matters too

Recipient behaviour is not the whole picture. Providers also scan what a message says when they decide whether it is spam, and certain words or phrasing can count against it.

Technical details matter

How you send is judged as well. Spam filters take into account:

• The number of connections you open to each receiving domain
• Email authentication (SPF, DKIM and DMARC, see DMARC)
• How you warm up new IP addresses (see IP Warm-Up)

You set these in your MTA or other sending infrastructure, whichever mail server software you run.

Important caveats

• The final decision belongs to the mailbox provider: the Inbox, the Spam folder, or refusing the message. Good practice makes your mail the kind a provider is willing to deliver to the Inbox, but no practice can promise Inbox placement.
• Expect to adjust in rounds, because improving deliverability is an iterative process. Each campaign adds to your sending history, and that data shows what to change next.

Related articles

• Two Worlds of Email Deliverability, on the principle of wanted and expected email in more depth
• IP Warm-Up
• Basic IP Allocation