As far as I know, there are no readers
fields in the Std mail template. However, to confirm it's not a reader's
field, you can open the target db in the Administrator client, turn on
full access admin and look in the all docs view to see if you see any additional
docs.
If you don't, you might want to look
at the target database in Notespeek, figure out where these missing documents
are being added. Suggestion: create a new empty target db, add few docs
until you reproduce the missing one, then check Notespeek.