Does anything different occur in the transmission of an email message where instead of typing in the address or looking through your contact list for the recipient, you just take an old message from them, hit reply, and remove all the old stuff so that it appears to be a new message.
Make sense?
The SMTP headers are pretty simple, and all the cases you describe should just fill in the email address of the intended recipient in the To field. The only difference might be that if you reply to one of their messages, it will include whatever identification they provided in addition to the address.
For example, my email client has both my name and email address, and it fills the From and Reply-to fields with something like “My Name <myaddress@domain.com>”. When you reply, this whole thing goes in the To field and the mailserver will parse the address out of the “<…>”. If you just type in their address, the name part will be missing (unless you type it). If you use an alias from your address book, it will use whatever address you associated with that alias, which could include the name info or not.
The only other difference would be the subject line, where hitting reply to one of their messages might add a "Re: " to their subject (depending on your email client). However, you can retype the subject line just like the body.
I do that all the time - easier than using an address list. About the only difference will be that you may wind up with a “References” header, so your recipient will be able to tell you were being lazy, or will have an irrelevant reference link in their display if their mailer uses the “References” header, and they retained a copy of the original message.
Turn on headers in your mailer, and you will see things like:
References: <567895.789543@somewhere.com>
This header says that the message refers to another message, indicated by the “Message ID”, namely the thing inside the angle brackets, which will contain the host name for the server where the message originated separated from a unique (within the host) identifier by an at sign. You may reference more than one message.
Don’t do this with usenet articles though. News readers are more likely to organize their displays by thread, making this extremely annoying.
The tag “In-Reply-To: [message ID]” often gets stuck into mail-headers by mailers. iirc, the tag was originally for Usenet only but I see it daily in headers of email that I send and receive. Of course, this is invisible to almost everybody.
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michael
Or “In-Reply-To”. The distinction is that “References” lists all of the messages that you are replying to if you reply to a reply, wherease “In-Reply-To” just gives you the one you actually replied to. A mail client may use either or both. “In-Reply-To” was a later addition, because mail and usenet clients were not very careful about maintaining any sort of order to the “References” header, and the new header made your immediate ancestor explicit.
For the record, I’m using the netscape client, which tacks on a cumulative “References” line. Outlook seems to like “In-Reply-To”, judging by mail I’ve received from people apparently using it.