The To: header lists one or more of the recipients of the mail message. Other headers, such as Cc:, also list recipients.

If the header of a mail message lacks recipient information (To:, Cc:, and Bcc: header lines), pre-V8.7 sendmail added an Apparently-To: header line and put the recipient's address from the envelope into the field of that header. Beginning with V8.7, the way a message with no recipients is handled is determined with the NoRecipientAction option (NoRecipientAction).