Lost Mail?

[D3s7]

I'm trying to figure out where mail from a specific host is going

If I TCPDump the interface I see the incoming connection from their mail server to mine however, that is the last location I find anything

None of the log files seem to have any record of a mail attempt and the mail is eventually returned to them undeliverable (give up retry)

My question is, can any advanced connection logging be enabled to see where this mail is going to?

Is there a tree of the process the mail goes through?

I always considered myself a somewhat advanced user however, this one has me stumped

[D3s7]

well.. progress

appears their server is causing rbl lookup errors

although, even if I disable the rbl's - the mail still isn't going anywhere

strange... strange... strange

[D3s7]

found it....

the SPF info was being blocked by their server which caused a failure