If Cached Exchange Mode is disabled in Outlook, messages appear to be sent successfully and appear in Sent Items but they are not actually sent.
This only occurs if Outlook for Office 365 or Outlook for Microsoft 365 is connected to Exchange Online. It does not occur with on-premise Exchange.
This issue occurs if PGP Tray is running. It occurs with both encrypted and unencrypted messages.
Symantec Encryption Desktop 10.4.2 and above.
Ensure that Cached Exchange Mode is enabled in Outlook. Cached Exchange Mode is enabled by default. To check that Cached Exchange Mode is enabled, please do the following:
Broadcom is committed to product quality and satisfied customers. This issue is currently being considered by Broadcom to be addressed in a forthcoming version or Maintenance Pack of the product. Please be sure to refer back to this article periodically as any changes to the status of the issue will be reflected here.