Unable to reply to a message due to Request Timeout error from Outlook client

When a user replies to a message from another user (internal user) she can't send the mails due to request timeout error. Normally, I'm telling them to go to address book and select the user instead of replying or selecting the stored history from their outlook just to send out the email but it's weird because how does the server doesn't recognize the email address since the user just replied to the recipient. Any explanation on this?