Tony,
Thanks for your input. I had considered the ISP blocking port 25 but I had already phoned them and they assured me that they don't block any ports. Today I decided to call again to double check and again the same answer but the person I spoke to suggested resetting the router to factory defaults and reconfigure it.
I did this and set it up again exactly as it was before but this time it worked.
Regards,
Lester Bennett