Mail delays between thin client and laptop versions of Outlook

We have had a few instances whereby users using outlook (2010) in Citrix are sending email but it’s not arriving until a much later date. Exchange 2010. Can anyone think of any reasons why this could be. We have both a thin client and laptop setup. It appears the delays happen when emails are sent via Citrix (from a thin client) then don’t send properly until the users logs onto the network with a Laptop. Have you ever come across this in your years as Citrix/Exchange admins.