[exim] Multiple messages per connection when queue-running

Hi all,

I have the problem of a very large mail queue which contain lots of
messages sent to the same remote address. Unfortunately the MX servers
for the remote domain are rather slow, and so when performing a queue
run in exim 4.66 the messages are trickling through very slowly as each
connection takes about 30 seconds to establish.

How can I persuade exim to send multiple envelopes to the same MX server
in the same SMTP connection to avoid the connection overhead? I've
found the queue_smtp_domains option, which appears to only be useful for
messages which are yet to be queued (and kills the possibility of
instant deliveries if there's no need to queue). How do I get normal
queued messages to add entries to the wait-remote_smtp hint database, so
that when delivering a message to the remote server it will hand the TCP
connection over to deliveries for other pending messages?

Enlightenment would be hugely appreciated :)

M.