Next time you get this error, please check *web mail* to see if that message is still there.
Not Outlook. Mail apps don't sync with each other, they sync with the mail server.
And since you use forwarding, please check web mail *for the exact account you are using in Aqua*.
Oh, and how many messages do you typically get per day? 10-20 or 30-40 or more?
Does the error seem to happen more often when accessing older messages?