Normally this can be solved by setting Outlook for a longer timeout period, this can be found in the Account settings, usually under Advanced options. Depending on exact version of Outlook the exact menu choices to get to this option screen will vary.
Another reason is newer AntiVirus programs can take longer to scan incoming and outgoing email on the client side, also resolved by increasing the timeout value in Outlook. Some have just disabled the AV scanning of emails, I do not recommend this as a course of action.
