If Encryption Desktop fails to send an encrypted message, you receive an NDR (Non-Delivery Report) from System Administrator with a Subject that begins Undeliverable.
When you click on the Send Again button to try to resend the message, a message is displayed stating that Outlook cannot resend the message.
The failed message does not appear in your Sent Items folder and therefore you have no way of retrieving the contents of the message you tried to send.
The text of the message in Outlook 2013 is as follows. It will be similar in other versions of Outlook:
Cannot resend this message. The nondelivery report does not contain sufficient information about the original message. To resend the message, open it in your Sent Items folder, and then on the Message tab, in the Actions group, click Other Actions, then click Resend This Message.
This is by design in Encryption Desktop 10.4.1 MP2 and above.
If an Outlook profile contains multiple Exchange mailboxes, Encryption Desktop does not attach the original message to the NDR.
If Encryption Desktop detects multiple mailboxes, it logs the following warning in the Encryption Desktop log. If you see this warning, the Send Again button will not work:
11:10:09 Email Warning MAPI Proxy: Outlook profile have multiple accounts.
Ensure that your Outlook profile only references one Exchange mailbox.
As stated in the Release Notes for Symantec Encryption Desktop 10.4.2 for Windows:
Symantec does not recommend creating multiple mailboxes under an Outlook profile because data loss might occur.
The data loss referred to occurs when the NDR does not contain the original message.