A simple thing. Wanted to add S/MIME encryption to email (I know, nothing much security wise – but still – playing around). Got a free cert at Comodo, imported it and sent a testmail to GMAIL. Gmail said it was “unsupported algorithm“. Turns out it’s Outlook that sets it as SHA1 and not SHA256 by default. Change it from SHA1 to SHA256 and GMAIL will like you again.
Change it in Archive – > Options -> Securitycenter -> Settings -> E-Mailsecurity -> Settings