v7.0.5 - Email Timeout Difficulties

Hi Guys, hoping I can draw on someones experience as I am pretty much stumped with a fault I have at present. I recently upgraded a v5.0.4 network to v7.0.5 and ever since the upgrade the client has been suffering from Microsoft Outlook/exchange problems. E-mail is occassionally timing out for a few seconds and items can be left in the outbox for long periods before it is able to send. The QoS is in place from v5 and was working fine and the network is not really congested at all. We have also had the client rule out a problem with the Exchange Server too. Have any of you guys heard of this difficulty before and how to eradicate it. I have already had a go with custom parameter keys in the remote options files but this did not help.