As I understand it, message tracking won't work because your appliance never accepted the message in the first place. As soon as the sending server said "message for X" your system replied "not here, friend" and ended the conversation.
A general principle when testing / experimenting: if you doubt your own test then rig a experiment to check it. In this case, send a mail from an external mailbox to an invalid address on your domain, then try grepping for the address you sent from. With grep, always start with a very short term that will give you too much information then refine it by trying progressively more complicated phrases to narrow the result down. If you think that a grep isn't working because of a new term you typed in, try grepping for something you can actually see at the end of the of the last result just to check that things are working as you expect.
Typical errors: case sensitivity; Rejected and rejected are not the same! Quoting can also be trying; in my case when logging on to my appliance using a very basic Telnet client I find that double quote marks delimit a phrase, but your environment may differ.
If you're not confortable working in the CLI and normally handle large text files elsewhere, why not simply FTP the mail.current log off to a convenient machine and examine it there instead? You can do a lot more with the implementation of grep in most other UNIX / Linux environments, and there are many other ways of handling large text files in other environments.
