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.
To fix this issue, install Cumulative Update 10 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.