I need to keep my regular email for longer than it is now. How do I extend the save period without archiving?

I choose not to make extra folders for my emails and would like to just keep them on my regular email for a longer period of time. I go back to February 2007 and find I cannot bring up an attachment and then a page comes up that says "Expired." How can I extend the length of time I can keep the emails I have already read?
I need to keep my regular email for longer than it is now. How do I extend the save period without archiving?
Your attachment with a page is all that has expired. Probably because it was time sensitive and the original poster has pulled it. The rest of your regular mail you can keep as long as you want. I have some in Yahoo Mail that is 10 years old now.