Why do the 'Segment Size', 'Reach', and 'Sent' numbers in my email campaign statistics differ?

It is normal for these numbers to differ — each one is measured at a different stage of the send.

• Segment Size — the number of devices matching your segment's filter conditions at the moment the audience is calculated. If you look at a single total, it also counts mobile and web devices that will never receive an email.
  • Tip: the segment builder has no "platform" condition, but the Estimated size line under the builder already splits the result per channel. Read the counter with the envelope icon ("Email addresses, including those unsubscribed") to get a realistic estimate for an email campaign.
• Reach — the number of users taken into the campaign. Note that in Customer Journey statistics Campaign reach is the "number of users who entered the journey (counted as non-unique users' entries)"; the separate Unique users metric is the deduplicated count. Compare the right pair before concluding something is wrong.
• Sent — the number of emails actually handed to the mail servers. This is the number the Email statistics Performance tab reports, next to Delivered, Opened unique, Opened total, Users clicked and Errors.

Why Sent differs from Segment Size

1. Pre-send filtering. Immediately before sending, invalid recipients are dropped: unsubscribed users, previously bounced addresses, and addresses marked as spam.
2. Several addresses per user. If one UserID has more than one valid email address, a message goes to each of them, which pushes Sent above the unique-user count.

Example. A segment contains 2,000 email devices. They belong to 1,750 unique users. Pre-send filtering removes 100 invalid addresses, leaving 1,900 — so Sent = 1,900, higher than the 1,750 unique users because some of them have two verified addresses.

Note: Sent is not Delivered. Bounces are subtracted afterwards, and for icloud.com, me.com and mac.com addresses the delivery rate can read low because Apple does not always return delivery confirmations.