I have few questions regarding this issue.
1. Did you try disabling the Firewall?
2. All the anti-viruses have an option “Scan email”, did you check that?
3. Are using a corporate email account?
4. Adding the domain & email address will not help, as the system is very smart. Its blocks the IP address. Try adding the sender’s IP address to your firwall.
If you not tried the above, then try & let me know the result.
Thanks for all your help. This is very sound advice and I am very grateful for the benefit of your expertise. Typing on a computer is the breadth of my technical prowess. I have tried all your suggestions and the only way we can seem to get round the problem is by the sender in question changing his e-mail provider. Fingers crossed. Thanks again.