Outlook prompts for a certificate every time and says 'Invalid Certificate - cannot sign or encrypt'

windows

Symptoms

After ActivClient is installed, Outlook keeps asking which cert to use, then refuses to sign/encrypt.

Likely cause

Outlook's E-mail Security options are set to sign / encrypt by default but Outlook does not know which cert to use.

Fix

Turn off the default sign/encrypt boxes: - Outlook 2013/2010: File > Options > Trust Center > Trust Center Settings > E-mail Security > uncheck the top four boxes. - Outlook 2007: Tools > Trust Center > E-mail Security > uncheck the top four boxes. - Outlook 2003: Tools > Options > Security tab > uncheck the top four boxes. Then pick sign/encrypt manually per message.

Sourced from militarycac.com. Rewritten for clarity.

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