my email address just accidently changed to an incorrect name?

Hi,

I recently sent an email to one of my friends and then next minute when he reply to my email it’s bounced back.

What happened was he rung me and told me that my email just changed to say for example. david@yahoo.com was my original email but when he (my friend) pressed reply (in Outlook for example) my email changed to david@yhaoo.com.

That is why he’s getting the bounced back, saying his messages could not be delivered to me due to my email (on his end) spelled incorrectly, I suppose.

I mean why would this change. This is weird

I am using Outlook 2007.

Probably to do with the reply settings on my Outlook settings, I suppose.

Help Please