Missing return email address! Please help!

I routinely receive emails with the only return address dispalyed being:
Sometimes, at a later time (within a day or two), I’ll recieve the email again with
the return address displayed from the actual sender, other times this doesn’t happen. What is causing this? There are emails I cant answer because I have no return address!
please help, if this continues I will have to other choice but to change servers.

do you have a FORM or something on your website that people are using to send you emails?

if so, this is what you would get if they did not enter a real email address in the email address field of the form.