Email forwarding: Limitations on reliability
Some Duke University users can set up automatic forwarding of their Duke email to off-site systems such as Gmail.
Duke cannot guarantee delivery of such messages. In particular, forwarding reliability may be affected by policies set by either the sender's email domain or by the recipient's email domain. This means that some messages may be received at Duke and forwarded to the destination but not displayed in the non-Duke destination mailbox. The way this works may change over time as various email providers change their policies. Duke has no control over this behavior.
If you decide to set up forwarding to an external provider and need to be sure that you don't lose any email, be sure to choose to "keep a copy" of forwarded messages in your Duke mailbox. Then, regularly review your Duke mailbox for any messages that were not delivered to your destination.
For more information, please see: KB0015771: Using Office 365 with a Different Mail Provider