Did you send the first network email?
As far as I know, yes. However, there are a few qualifications. Network should be included because there were many earlier instances of email within a single machine. Computer networks, in any real sense, didn't exist until the ARPANET was built starting in 1969. Dick Watson proposed a form of email in July 1971 (RFC 196). I don't think that was ever implemented. It differed in that the mail was directed to numeric mailboxes. RFC 196 also suggests that the final product would be a printer output (i.e. ink on paper). SNDMSG sent messages to named individuals (computer users).
Why did you do it?
Mostly because it seemed like a neat idea. There was no directive to "go forth and invent email". The ARPANET was a solution looking for a problem. A colleague suggested that I not tell my boss what I had done because email wasn't in our statement of work. That was really said in jest because we were, after all, investigating ways in which to use the ARPANET.
Why did you choose the at sign?
The primary reason was that it made sense. at signs didn't appear in names so there would be no ambiguity about where the separation between login name and host name occurred. (Of course, this last notion is now refuted by the proliferation of products, services, slogans, etc. incorporating the at sign.) The at sign also had no significance in any editors that ran on TENEX. I was later reminded that the Multics time-sharing system used the at sign as its line-erase character. This caused a fair amount of grief in that community of users. Multics used IBM 2741 terminals which used EBCDIC character coding. They did not have a "control" modifier key and didn't have many (any?) non-printing characters beyond space, backspace, tab, and return. The designers of Multics were constrained to using printing characters for line-editing.
What was the first message?
The first message of any substance was a message announcing the availability of network email. The exact content is unknown, but it gave instructions about using the at sign to separate the user's name from his host computer name.
Did you receive any rewards, patents, etc.?
Not unless you consider the current interest in the origins of email a reward.
What were the early uses of email?
The early uses were not terribly different from the current uses: The exceptions are that there was only plain text in the messages and there was no SPAM.