One might very well say that a mail header is a request (in that the
user enters it and the SMTP server for the given domain needs to look
it up to return success or failure), so that it's slots would fall into
the "query" category. And naturally, one would like to have the content
of the message headers and that of the smtp commands be subject to the
same rules, now that 282{1,2} agree on the mailbox definition.
However, the POV that the message is stored and it's addresses will be
subject to queries (e.g. by user filtering or searching) isn't way off,
too. There's also the argument that what the server looks up is the
argument given to SMTP's RCPT TO command, not what's in the header
fields.