Foundations of Email Deliverability
The core principle of deliverability — sending wanted and expected email — and how mailbox providers measure it.
The foundation of good email deliverability is sending email that is:
- Wanted – meaning you offer something of value to the recipients.
- Expected – meaning your recipients remember subscribing or remember their business relationship with your brand.
How mailbox providers evaluate your email
ISPs and mailbox providers don't have a crystal ball that tells them if the recipients wanted or expected the email — they have something better: statistical information on how tens of thousands of their users interacted with your email. The most important signals are the complaint rate and the engagement rates of the email. They know:
- What percentage of people clicked "this is spam" on your email
- What percentage read your message
- What percentage deleted it without reading
- How long people spent reading it
All of this goes into their algorithms, and they infer whether your email is wanted and expected.
Content matters too
Mailbox providers also look at the content of the message to try to discern whether it is spam. Some keywords or language in email can contribute to spam filtering.
Technical details matter
Technical sending details also matter to spam filters, including:
- How many connections to open toward each receiving domain
- Email authentication (SPF, DKIM, DMARC — see DMARC)
- Warm-up schedules for new IP addresses (see IP Warm-Up)
These are configured at the MTA / sending-infrastructure level, regardless of which mail server software you use.
Important caveats
- It is the mailbox providers who decide whether your email lands in the Inbox, the Spam folder, or is rejected outright. Best practices are designed to help you send email that providers want to put in the Inbox — they don't guarantee Inbox placement.
- Deliverability enhancement is an iterative process. As sending history accumulates, you gain more data, which informs further adjustments.
Related
- Two Worlds of Email Deliverability — a deeper treatment of the wanted/expected principle
- IP Warm-Up
- Basic IP Allocation