Email security issues

We use Esign to make it easy for customers to sign contracts for our services. More and more we are getting complaints that they are unable to click the link in the email. This seems to down to either ISP security or users anti virus making the link non clickable.

The ISPs I know about so far are: Yahoo!, Claranet, Sky

Trying to talk some of these people through copying an pasting the link in a web browser is like I've asked them to do open heart surgery.

Is there any other way of them getting to sign the contracts? I though a code that they could type into an easily searchable web site might provide the answer.