[exim] defer spam condition, if timeout occurs

hi,

i've set up exim to spamscan all mails in the data ACL
and tag every message that's smaller than 1M:

  warn    message   = X-Spam-Score: $spam_score\n\
                       X-Spam-Score-Int: $spam_score_int\n\
                       X-Spam-Bar: $spam_bar\n\
                       X-Spam-Report: $spam_report
           condition = ${if < {$message_size}{1M}}
           spam      = spamassassin:true

whenever the load on my system gets to high, the spamd-connection will
timeout and the mail passes. as i want these messages to be rejected
temporary, i want to know, if there's a better way to defer the message
than to add the following:

  defer  log_message    = problem with spamd timeout
    condition     = ${if < {$message_size}{1M}}
    spam          = spamassassin:true
         condition       = ${if !def:spam_score}

this second one tries to figure out, if $spam_score is empty and assumes
there was a timeout.
btw: is the second 'spam = spamassassin:true' is nessassary, or may i
refer to the cached $spam_score from the first one?

cheers
marko