Assume that you use Microsoft Outlook 2010 or Microsoft Outlook 2013 in Online Mode to delay deliver an email message in a Microsoft Exchange Server 2013 environment. When you open the message in the Outbox folder, the message is in Read-Only status, and the Send button isn't available for resending. However, it works fine if you reopen the message.
This is a known issue. It occurs because the UNSENT flag is marked incorrectly for the delayed delivery message.