To

RFC 5322 ● 3.6.3. Destination Address Fields

The destination fields of a message consist of three possible fields, each of the same form: the field name, which is either "To", "Cc", or "Bcc", followed by a comma-separated list of one or more addresses (either mailbox or group syntax).

to  = "To:" address-list CRLF

cc  = "CC:" address-list CRLF

bcc = "BCC:" [address-list / CFWS] CRLF

The destination fields specify the recipients of the message. Each destination field may have one or more addresses, and the addresses indicate the intended recipients of the message. The only difference between the three fields is how each is used.

To

The "To:" field contains the address(es) of the primary recipient(s) of the message.

When a message is a reply to another message, the mailboxes of the authors of the original message (the mailboxes in the "From:" field) or mailboxes specified in the "Reply-To:" field (if it exists) MAY appear in the "To:" field of the reply since these would normally be the primary recipients of the reply. If a reply is sent to a message that has destination fields, it is often desirable to send a copy of the reply to all of the recipients of the message, in addition to the author. When such a reply is formed, addresses in the "To:" and "Cc:" fields of the original message MAY appear in the "Cc:" field of the reply, since these are normally secondary recipients of the reply. If a "Bcc:" field is present in the original message, addresses in that field MAY appear in the "Bcc:" field of the reply, but they SHOULD NOT appear in the "To:" or "Cc:" fields.

Note: Some mail applications have automatic reply commands that include the destination addresses of the original message in the destination addresses of the reply. How those reply commands behave is implementation dependent and is beyond the scope of this document. In particular, whether or not to include the original destination addresses when the original message had a "Reply-To:" field is not addressed here.

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BCC ● Blind Carbon Copy

CC ● Carbon Copy

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To

Field Count Notes
BCC  0-1
CC  0-1
Comments  0-*
From   1
In-Reply-To  0-1
Keywords  0-*
Message-ID  0-1
Orig-Date   1
References  0-1
Reply-To  0-1
Sender  0-1
Subject  0-1
To  0-1