Classic ASP CDOSYS and Exchange CAS server

I'm having an issue where my comapany are migrating our exchange to a new CAS server.

We have several pages on our website using classic asp that sends email using CDOSYS which are working fine with the current exchange server.

When I swap over to the new CAS server I get a weird thing happening. The emails are sent instantly as they should but the script itself takes just over 30 seconds to complete. No errors or anything there's just a very long delay.

This is my code I use for testing this.

Set myMail = CreateObject("CDO.Message")
myMail.Subject = "A test email"
myMail.From = "A valid email address"
myMail.To = "A valid email address"
myMail.TextBody = "This is a test email."
myMail.Configuration.Fields.Item _
("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
myMail.Configuration.Fields.Item _
("http://schemas.microsoft.com/cdo/configuration/smtpserver") = "1.2.3.4"
myMail.Configuration.Fields.Item _
("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25 
myMail.Configuration.Fields.Update
myMail.Send
set myMail = Nothing

I can't work out why this is, is it something with the code or something with the new exchange server.

I've played around with smtpconnectiontimeout but makes no difference.

objCDO.Configuration.Fields.Item _
("http://schemas.microsoft.com/cdo/configuration/smtpconnectiontimeout") = 5

The web server is running Windows 2008 with IIS7.

Anyone experienced something similar?