Is it possible to force all email from all users of a domain to have a signature attached to ALL mail sent. I can see how this can be done when clients use webmail, but I am thinking the situation where clients use a mail client such as outlook, thunderbird , phone etc, where client has no signature set up on their email client, so that as mail is sent the signature is appended to the email at the server level, before being sent?

I may be missing something in the documentation ? But this does not look possible unless using webmail?