Will the info I want about an incoming email survive in the envelope?

I have a situation where emails bound for several recipients will arrive in one mailbox. I want t开发者_开发知识库o go through and deal with them according to who they were sent to, but I'm not sure I will always have the correct info.

Suppose I have one recipient fred@domain1.com . if someone bcc's him on an email, I know that fred will not see any To: field in his email BUT I will be able to find fred@domain1.com in the envelope. No problem.

Now, what if fred has some email forwarding service. Or lets say he asks gmail to forward a copy of every email he receives at gmail on to his fred@domain1.com address.

The original message that was sent to him may have said fred@gmail.com in the envelope, and will still say it in the To: but once gmail has forwarded it, can I be sure I will always find fred@domain1.com in the envelope for the "final leg" of its journey ? fred@gmail.com it would be no use to me, you see.

Or will I find (somewhere in the envelope) references to both the gmail AND the domain1 addresses ?

TIA

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I believe MTAs add Delivered-To or X-Forwarded (or similar, depends on the MTA) for the forwarded message. To: headers usually stay as-is.