Sales Loves Marketing

What is Email Delivery?

Email delivery is the moment a receiving mail server accepts a message, confirmed by an SMTP "250" reply. It says nothing about whether the email lands in the inbox or the spam folder; that is deliverability.

By Sales Loves Marketing teamOriginally written by Joran HofmanUpdated

Email delivery is the point where the receiving mail server accepts your message. Technically, an email is delivered when the receiving server answers with an SMTP "250" reply after the message data is sent. Whether it then reaches the inbox or the spam folder is a separate question, called deliverability.

How does email delivery work?

Email moves between servers using SMTP, the Simple Mail Transfer Protocol, defined in RFC 5321.

  1. Your sending server, a mail transfer agent, connects to the recipient's mail server.
  2. It says who the message is from and who it is for. The receiving server can accept or reject each step.
  3. It sends the message content.
  4. The receiving server replies with a code.

The reply code decides the outcome:

  • 2xx (success). Code 250 means "Requested mail action okay, completed." RFC 5321 says that when a server sends this after receiving the message, it "is accepting responsibility for delivering or relaying the message."
  • 4xx (temporary failure). The server refused for now, for example because it is busy or is rate-limiting you. Your server will retry. This is often reported as a soft bounce.
  • 5xx (permanent failure). The server refused for good, for example because the address does not exist. This is a hard bounce.

When is an email really "delivered"?

People use the word in three ways:

  1. The server accepted it. This is the technical meaning, and what most email platforms report as "delivered."
  2. It was filed in the recipient's mailbox, in the inbox or a spam folder.
  3. The recipient opened it.

Think of a parcel sent to an office building. The front desk signs for it (accepted). Someone carries it to the right desk (filed). The person opens it (read). Only the first step is "delivery" in the technical sense.

How do you measure delivery?

Most email platforms report a delivery rate:

Delivery rate = (emails sent − bounces) ÷ emails sent

A low delivery rate usually points to list problems, such as old or mistyped addresses, or to a server rejecting your mail because of authentication or reputation. See sender score.

How do you improve delivery?

  • Clean your list. Remove hard bounces straight away and confirm new sign-ups with opt-in.
  • Authenticate your domain. Google's sender guidelines require SPF or DKIM for all senders, and SPF, DKIM and DMARC for senders of more than 5,000 messages a day to Gmail accounts. See DMARC.
  • Set up reverse DNS. Google requires the sending IP address to have a PTR record that resolves to a hostname.
  • Watch spam complaints. Google advises keeping the spam rate in Postmaster Tools below 0.10% and never reaching 0.30%.

For why this matters beyond the technical layer, see the importance of email deliverability.

Frequently asked questions

What does "delivered" mean in email marketing reports?

In most email platforms, delivered means the receiving server accepted the message instead of bouncing it. It does not mean the email reached the inbox or was opened.

What is the difference between delivery and deliverability?

Delivery asks whether the receiving server accepted the email. Deliverability asks where it went after that, the inbox, a promotions tab or spam. You can have a high delivery rate and still have poor deliverability.

What does a 250 OK SMTP response mean?

In the SMTP standard, RFC 5321, reply code 250 means "Requested mail action okay, completed." When a server sends it after receiving the message data, it accepts responsibility for delivering or relaying the message.