[exim] rate limit outgoing emails?

Hello,

We have a mailing list in the school here (generated by ezmlm via
qmail) and there are email addresses on that list outside of the
district here.

We have about 500 people on the list (teachers with personal/other
email addresses) and this mailing happened to have a 7MB pdf attached
to it.

Exim did its job, but it killed our little pipeline that we have to
the outside until it was done sending.

Other than altq'ing the outbound smtp, is there some exim options for
controlling or ratelimiting sending?

qmail sends to exim as one email per recipient, so the default exim
smtp_accept_queue_per_connection never gets triggered..

I'd like to make the rule(s) generic enough that they would apply to
all future mailings from this list and not from this list..

so something like:

"If the message is over 2Mb, add some sort of random time" - or
something like that.

Clues and advice appreicated

This is the version of exim on the outbound server.

exim -bV
Exim version 4.69 #0 (FreeBSD 6.3) built 28-Mar-2008 16:31:25
Copyright (c) University of Cambridge 2006
Probably Berkeley DB version 1.8x (native mode)
Support for: crypteq iconv() use_setclassresources PAM Expand_dlfunc
OpenSSL Content_Scanning Old_Demime
Lookups: lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmnz
dnsdb dsearch nis nis0 passwd
Authenticators: cram_md5 dovecot plaintext spa
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp
Fixed never_users: 0
Size of off_t: 8
Configuration file is /usr/local/etc/exim/configure