[exim]  421 Too many concurrent connections from this client

We have a Kayako helpdesk, when e-mails come in from customers notifications are sent out by Kayako to a number of staff whose mailboxes are hosted on Rackspace mail servers.

Rackspace policy is 5 connections to their mx1 and 3 connections to mx2.

I noticed a large number of queued messages in Exim - when looking in Exim logs I can see many lines 2012-10-13 20:06:56 1TN72s-0007Cw-1l SMTP error from remote mail server after initial connection: host mx2.emailsrvr.com [173.203.2.32]: 421 Too many concurrent connections from this client.

For our requirements if we send 1 email every 10 seconds or so, this would be OK.

Messages to all other servers should go through a normal rates, only mx1.emailsrvr.com and mx2.emailsrvr.com should have this connection limit policy applied.

Is this possible?

Many thanks,

Chris