Outlook 2003: E-mail messages take a long time to send and receive when you use an Exchange 2010 mailbox

This problem occurs because Exchange Server 2010 does not issue UDP notifications to Outlook 2003. Therefore, Outlook 2003 cannot register to receive UDP notifications from Exchange Server 2010. Additionally, Outlook 2003 is not notified about changes that are made to a folder until Outlook 2003 polls the server for these changes. The default polling interval for Outlook 2003 is about 60 seconds. This is why you see delays that last approximately 1 minute in these folder actions

https://support.microsoft.com/default.aspx?scid=kb;en-us;2009942