Opening Links in Received Emails

Hello,
I cannot open links received in emails. Every time I click on the link, I receive an Error message telling me that I can't open the link. I used to be able to open links without a problem, but I don't think there have been any preference settings changes which would affect this. I have unsuccessfully searched through Mail Preferences trying to figure out how to fix this problem. Any suggestions on how to fix this problem would be greatly appreciated.