[exim] How to configure to ignore responses to autoresponder…

Hi, I have a problem where a user sets a vacation message which
autorespond to an incoming email. In some situations, the sender's
mailbox is full when the auto message reaches him/her, or so far
mostly a spam with a spoofed email address that's already invalid.

What happens next I believe is a mail loop. The "sender" auto replies
with a "cannot deliver, mailbox is full or user is invalid". Then my
user's vacation message respond to that, and so they pingpong until
even my user gets a full inbox which then seems to generate two
responses, one for vacation and one for mailbox full. Things go south
rapidly from there as each bounce causes the email to grow bigger and
bigger, making spamassasin/clamav spend more and more time processing
them, as well as exim requeuing them for another attempted delivery.

Based on what I've found online, I already set the option in exim to
check for received_headers_max. Unfortunately, in this case, it seems
that the other server strips all headers so this limit is never hit.

The only thing I can see from looking at the bouncing emails that
might be a clue is the returned mail gets the whole email subject
appended with a newline char included. However, I can't figure out how
to get exim to drop any emails with more than 1 \n in the subject or
any \n. Since I don't think in the first place there should be any
newline chars in a normal email subject.

Any advise on how I can achieve this or is there a better way to catch
this sort of situation?