> On Apr 18, 2018, at 2:40 PM, Erwin <joc...@gmail.com> wrote:
> 
> Hi,
> 
> This may be old hat to some, but staring at the RFCs (specifically 2821) the 
> only conclusion I see is that Microsoft is (or at least *.outlook.com servers 
> are) violating the format of the Received headers here:
> 
> Received: from mta.email.thinkgeek.com (66.231.88.32) by 
> SN1NAM04FT019.mail.protection.outlook.com (10.152.88.152) with Microsoft SMTP 
> Server (version=TLS1_0, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P384) id 
> 15.20.675.14 via Frontend Transport; Mon, 16 Apr 2018 16:33:42 +0000
> 
> Should that IPv4 literal be enclosed with "[" and "]" tokens, either as 
> ([a.b.c.d]) or (hostname [a.b.c.d])?

According to the BNF yes, probably. In practice parsing Received headers is 
more about heuristics than a formal grammar, especially with comments^Wanything 
inside parentheses.



   Time-stamp-line  = "Received:" FWS Stamp <CRLF>

   Stamp          = From-domain By-domain Opt-info [CFWS] ";"
                  FWS date-time

   From-domain    = "FROM" FWS Extended-Domain

   By-domain      = CFWS "BY" FWS Extended-Domain

   Extended-Domain  = Domain /
                    ( Domain FWS "(" TCP-info ")" ) /
                    ( address-literal FWS "(" TCP-info ")" )

   TCP-info       = address-literal / ( Domain FWS address-literal )
              
   address-literal  = "[" ( IPv4-address-literal /
                    IPv6-address-literal /
                    General-address-literal ) "]"

Cheers,
  Steve


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