rfc2822 April 2001

4.5.3. Obsolete destination address fields

obs-to          =       "To" *WSP ":" address-list CRLF

obs-cc          =       "Cc" *WSP ":" address-list CRLF

obs-bcc         =       "Bcc" *WSP ":" (address-list / [CFWS]) CRLF
   When multiple occurrences of destination address fields occur in a
   message, they SHOULD be treated as if the address-list in the first
   occurrence of the field is combined with the address lists of the
   subsequent occurrences by adding a comma and concatenating.