This is an extension of milter-protocol.txt document which attempts to document
changes made to the milter protocol.

What is described here together with milter-protocol.txt is believed to be
milter protocol version 6 as understood by sendmail 8.15.2 and libmilter 1.0.1.

----------------------
Protocol negotiation

Actions:

0x00000001    SMFIF_ADDHDRS         Add headers (SMFIR_ADDHEADER) [v1]
                                      *and insert headers (SMFIR_INSHEADER)*
0x00000002    SMFIF_CHGBODY         Change body chunks (SMFIR_REPLBODY) [v1]
0x00000004    SMFIF_ADDRCPT         Add recipients (SMFIR_ADDRCPT) [v1]
0x00000008    SMFIF_DELRCPT         Remove recipients (SMFIR_DELRCPT) [v1]
0x00000010    SMFIF_CHGHDRS         Change or delete headers (SMFIR_CHGHEADER) [v2]
0x00000020    SMFIF_QUARANTINE      Quarantine message (SMFIR_QUARANTINE) [v2]

0x00000040    SMFIF_CHGFROM         Change envelope sender (SMFIR_CHGFROM)
0x00000080    SMFIF_ADDRCPT_PAR     Add recipient with ESMTP args

0x00000100    SMFIF_SETSYMLIST      Send set of macros needed

Protocol flags:

0x00000001  SMFIP_NOCONNECT     Skip SMFIC_CONNECT [v1]
0x00000002  SMFIP_NOHELO        Skip SMFIC_HELO [v1]
0x00000004  SMFIP_NOMAIL        Skip SMFIC_MAIL [v1]
0x00000008  SMFIP_NORCPT        Skip SMFIC_RCPT [v1]
0x00000010  SMFIP_NOBODY        Skip SMFIC_BODY [v1]
0x00000020  SMFIP_NOHDRS        Skip SMFIC_HEADER [v1]
0x00000040  SMFIP_NOEOH         Skip SMFIC_EOH [v2]

0x00000080  SMFIP_NR_HDR        No reply for SMFIC_HEADER
0x00000100  SMFIP_NOUNKNOWN     MTA should not send unknown commands
0x00000200  SMFIP_NODATA        MTA should not send DATA
0x00000400  SMFIP_SKIP          MTA understands SMFIS_SKIP
0x00000800  SMFIP_RCPT_REJ      MTA should also send rejected RCPTs
0x00001000  SMFIP_NR_CONN       No reply for SMFIC_CONNECT
0x00002000  SMFIP_NR_HELO       No reply for SMFIC_HELO
0x00004000  SMFIP_NR_MAIL       No reply for SMFIC_MAIL
0x00008000  SMFIP_NR_RCPT       No reply for SMFIC_RCPT
0x00010000  SMFIP_NR_DATA       No reply for SMFIC_DATA
0x00020000  SMFIP_NR_UNKN       No reply for SMFIC_UNKN
0x00040000  SMFIP_NR_EOH        No reply for SMFIC_EOH
0x00080000  SMFIP_NR_BODY       No reply for body chunk
0x00100000  SMFIP_HDR_LEADSPC   Header value leading space **
0x10000000  SMFIP_MDS_256K      Max DATA size = 256K (?)
0x20000000  SMFIP_MDS_1M        Max DATA size = 1M (?)

[**] From libmilter docs: Indicates that the MTA can send header values with
leading space intact.  If this protocol step is requested, then the MTA will
also not add a leading space to headers when they are added, inserted, or
changed.

----------------
Command codes

**

'K'     SMFIC_QUIC_NC       QUIT but new connection follows

???

**

'T'     SMFIC_DATA          DATA

Called when the client uses the DATA command.

**

'U'     SMFIC_UNKNOWN       Any unknown command (?)

???

----------------
Response codes

**

'2'     SMFIR_ADDRCPT_PAR   Add recipient with ESMTP args

char    args[][]            Array of strings, NUL terminated (address at index 0)
                            args[0] is recipient, with <> qualification.
                            args[1] and beyond are ESMTP arguments, if any

**

'e'     SMFIR_CHGFROM       Replace envelope from address

char    args[][]            Array of strings, NUL terminated (address at index 0)
                            args[0] is sender, with <> qualification.
                            args[1] and beyond are ESMTP arguments, if any

**

'f'     SMFIR_CONN_FAIL     Cause a connection failure

???

**

'i'     SMFIR_INSHEADER     Insert header at a specified position (modification action)

uint32  index           Index into header list where insertion should occur
char    name[]          Name of header, NUL terminated
char    value[]         Value of header, NUL terminated

**

'l'     SMFIR_SETSYMLIST    Set list of symbols (macros)

???

**

's'     SMFIR_SKIP          Do not send more body chunks

A milter has received sufficiently many body chunks to make a decision, but still
wants to perform message modification functions that are only allowed to be
returned in response to SMFIC_BODYEOB.