When mutt is run with the standard input not attached to a terminal
and without an e-mail address as an argument, it sends a mail, while
the help text requires at least an address in argument (which makes
sense). For instance:
echo foo | mutt
When there are no default recipients, one gets an error, but when
there are default recipients (via my_hdr), a mail is really sent,
with the contents of the standard input (in the example, "foo").
As sending a mail is not expected in this case, this might be a
security issue (even if the recipient addresses belong to the user).