In Microsoft Outlook 2016 for Mac, you compose a new encrypted email message addressed to a user in the global address list (GAL). When you click Send, you receive the following error message and the email does not send:
Message could not be saved.
This issue may occur when the email message recipient's name in the GAL differs from the name on their certificate due to case sensitivity.
To work around this issue, manually enter the full email address of the recipient instead of selecting them from the GAL.