I installed a s/mime digital signature on my wife’s iMac because I thought it was cool. However she turned off the ‘always sign’ option when correspondents using Outlook Express complained due to limitations in how OE deals with s/mime.

Recently I wanted to make sure it was still working so I sent a test email to my account from hers, and while Outlook 2003 thought everything was fine, Apple Mail 2.0 complained with ‘Unable to verify digital signature.’ After searching for a clue, I decided to check the settings in Entourage and found the solution. If you want the option to send digital signatures via the pull down menu, in the account options you must leave ‘Include my certificate when sending signed messages’ checked. Without this Apple Mail will complain.