There are too many stupid people on these boards. "E-mail spoofing" has been, at various times, described as having a weak password broken, logging in to the wrong account, using Yahoo and getting hit by a spambot, or having a GMail account with a period in their address and receiving someone else's mail mistakenly, among other things.If you don't know what it is, chances are you won't know how to stop it.
Setting a reply-to address is not e-mail spoofing, as it does not necessarily appear to be from you. If the e-mail address that actually sends the e-mail is similar to the legitimate address, you should get in touch with your e-mail service. You will not be able to stop this from the client end of things.
He asked an honest question. Maybe since you are one of the informed/enlightened you could turn a light bulb on and contact the originating mail relay allowing this to happen? Maybe contacting Gmail regarding it?If you don't know what it is, chances are you won't know how to stop it.
It's when someone uses another email address but sends emails with the reply to address as your email. People then receive emails which look like they are coming from you but are in fact sent from the spammers address.