S/Mime compability when NOT using your own laptop

Hi

I have a question about S/MIME when I´m not using my own laptop, should this work?

For exampel if I use my PC at home I don´t have my corporate certificate and of course I can´t sign/encrypt my email. Is it possible to solve this in anyway?

Another exampel if I have a PC from my work but it´s not part of our domain,  it´s part of a remote user Domain, no trust with corporate domain. We have a CA in remote user Domain is it possible to get Exchange to trust our certificates?

As it is now I receive following error:

Outlook Web App needs your digital ID to save a draft of a message that has a digital signature. If your digital ID is on a smart card, insert the card in the card reader and try to save the message again.


If your digital ID isn't trusted by the Exchange server, you can't use it to sign messages.

Thanks in advance

Mikael