[exim] handling failures in pipes

I'm delivering mail to cyrus in a pretty standard way:

virtualuser_delivery:
     driver = pipe
     user = cyrus
     group = mail
     message_prefix = ""
     message_suffix = ""
     log_fail_output
     return_output
     envelope_to_add = true
     command = "/usr/lib/cyrus/deliver -r $return_path $local_part@$domain"

however....If cyrus is down for any reason, mail sent to users will be
bounced with a message:

couldn't connect to lmtpd: Connection refused
421 4.3.0 deliver: couldn't connect to lmtpd

I would rather mail be accepted and stored locally untill cyrus is
backup, bouncing the mail just because cyrus is down is not an option.

How might I achieve this?

Thanks,


--
Ian P. Christian ~ http://pookey.co.uk