unable to sign or encrypt message certificate not installed
It turns out that, for some reason, the option to sign outgoing messages had been turned on by accident - without any client certificate installed for this feature to actually work.
To fix it - there are two options:
1) Install a client certificate if you actually want to sign outgoing emails.
2) Turn signing off.
Hardly anyone signs emails and I am guessing that most people who want to use it will already know how to!
As for turning signing off, it is pretty straight forward.
1. Go to Email settings (Easiest is in the email client, click the menu button and go to settings).
2. Go down to Security Options
3. Untick sign.
Hope this helps!