This is a proposal, not the real IANA registry! For more info see Mail and Netnews Header Registration Procedure.

IANA Registry of e-mail headers

Last update: 15 August 1997.

Description Name Reference, status

Special Usenet News actions and a normal article at the same time. Also-Control: son-of-RFC1036 [21], non-standard, only in Usenet News, not in e-mail

Controls whether this message may be forwarded to alternate recipients such as a postmaster if delivery is not possible to the intended recipient. Default: Allowed. Alternate-Recipient: RFC 1327, not for general usage.

Inserted by Sendmail when there is no "To:" recipient in the original message, listing recipients derived from the envelope into the message heading. This behavior is not quite proper, MTAs should not modify headings (except inserting Received lines), and it can in some cases cause Bcc recipients to be wrongly divulged to non-Bcc recipients. Apparently-To: Non-standard, discouraged, mentioned in RFC 1211.

Name of the moderator of the newsgroup to which this article is sent; necessary on an article sent to a moderated newsgroup to allow its distribution to the newsgroup members. Also used on certain control messages, which are only performed if they are marked as Approved. Approved: RFC 1036: 2.2.11, not standardized for use in e-mail.

Description	Name	Reference, status
Special Usenet News actions and a normal article at the same time.	Also-Control:	son-of-RFC1036 [21], non-standard, only in Usenet News, not in e-mail
Controls whether this message may be forwarded to alternate recipients such as a postmaster if delivery is not possible to the intended recipient. Default: Allowed.	Alternate-Recipient:	RFC 1327, not for general usage.
Inserted by Sendmail when there is no "To:" recipient in the original message, listing recipients derived from the envelope into the message heading. This behavior is not quite proper, MTAs should not modify headings (except inserting Received lines), and it can in some cases cause Bcc recipients to be wrongly divulged to non-Bcc recipients.	Apparently-To:	Non-standard, discouraged, mentioned in RFC 1211.
Name of the moderator of the newsgroup to which this article is sent; necessary on an article sent to a moderated newsgroup to allow its distribution to the newsgroup members. Also used on certain control messages, which are only performed if they are marked as Approved.	Approved:	RFC 1036: 2.2.11, not standardized for use in e-mail.