[Exim] Autoreplies with Subject "Re: $h_subject:" dangerous?

Hello,

I just see that the Exim filter specification 3.26 says:

  if personal then
    mail
     to $reply_address
     subject "Re: $h_subject:"
     file $home/vacation/message
     once $home/vacation/once
     once_repeat 10d
  endif

I consider that dangerous, because mailing lists often verify addresses
by sending a mail with a key in the subject. Using the vacation
responder above, anybody could subscribe its user to such mailing lists.
Besides being annoying, spammers would get opt-in confirmations for their
"newsletters".

Michael