There are a few possible causes if the number of addresses specified is different from the number for delivery.
Unusable addresses are included
- Contacts with blank Email1 field
- If you're sending to a list: Open the list you created and check for any contacts with a blank Email1 field.
- If you're using a tag to send: Search and download the list using the tag name as the condition, and and check for any contacts with a blank Email1 field. See here for how to search contacts by tag name.
- Contacts set as private or with private fields
- If the contact's owner isn't the user doing the mailing, there may be a difference in the number of viewable contacts.
- Contacts are still being digitized.
- The domain of the email address of the sender (contact owner) isn't the registered domain when doing one-to-one mails.
Filtered addresses are included
- There are recipients on the unsubscribed or blocked lists.
- Contacts included are in the undeliverable list
- If the delivery status is "Unknown user", "Unknown host", "Rejected", or "Unidentified error", the contact will be automatically added to the undeliverable filter and excluded from future deliveries.
- 3. There are duplicate emails.
- Multiple contacts of the same person are included
- Those other than the latest contact will be excluded.