The email header field that lists the main recipients of a message is To.
The To: field contains the address or addresses of the message’s primary recipients. More than one address can appear there, separated according to email’s address-list syntax. Email clients normally display these recipients prominently because they are the people expected to read or act on the message.
To: is different from Cc: and Bcc:. Cc, or carbon copy, identifies secondary recipients whose addresses are visible to other recipients. Bcc, or blind carbon copy, hides those recipients from the ordinary message header shown to others. Subject: describes the topic, From: identifies the claimed author, and Reply-To: can specify a different address for responses.
A useful technical wrinkle is that To: is part of the message header, not necessarily the complete delivery instruction. SMTP separately carries envelope recipients through RCPT TO commands, so a message can be delivered to an address that is absent from To:, such as a Bcc recipient. RFC 5322 defines the modern Internet message format and its header-field syntax.