[Exim] Exim 4 and Cyrus quotas

I am using Exim 4 with Cyrus and Spamassasin. After doing some reading,
I've read that Exim usually bounces back quota errors. It looks like the
quota errors look like this in the mainlog:

Child process of cyrus_transport transport returned 75 (could mean
temporary error) from command: /usr/cyrus/bin/deliver

The weird thing is that some messages aren't being bounced right
away. They are still sitting in the queue.

Here is the transport line in the configure file:

cyrus_transport:
   driver = pipe
   command = /usr/cyrus/bin/deliver \
             -m ${substr_1:$local_part_suffix} -- $local_part
   user = cyrus
   group = mail
   return_output
   log_output
   message_prefix =
   message_suffix =

Is there something I am missing?

Thanks,

Doug