I use CACert.org for my internal servers as well and was getting a BAD Signature error when sending via secure SMTP.

To fix the problem I simply installed the CACert root certificate on my N900 by downloading the Class 1 PKI Key in DER format from CACert.org by selecting "Save as" in MicroB and then opening it with file manager which prompted me to install it and gave me the options for it's use i.e. server, WLAN, or email ( I selected all three ).

Tested it by sending an email which went no problems without prompting about the certificates "BAD Signature".

Hope this info helps.

Cheers,

w0rkRB