Just a quick query on the use of the User-Agent mail header with RC. Which RFC is this from?

I thought it was a HTTP header and User Defined Fields (X-$) were defined for mail in RFC822 which for some reason don't get a mention in RFC2822 which succeeds it. Deprecated?

Is this the right header to use for identification of the software used to send the mail? I don't know much here, any info on this appreciated

Chris