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 to 10.5 MP1.
Upgrade to release 10.5 MP2 or above where this issue is resolved.
If you cannot upgrade, 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: