I am showing ad as my mail signature through adserver and it's working fine. When i send to someone and then other will receive mail and suppose he choose to download images in mail then google will call to third party url and my ad will display in mail as signature but when that user will forward/discard/reply mail then google is requesting signature image again. Can you tell me what reasons will be for this?
Well, are you sure the image is attached (attachment) or is it just a link to your website? <img src=..>
Yes, its <img src='http://www.example.com/getimage/keyword/realestate/30/12/34/34343'>, not as attachment. See attachment, it is screen shot of my email signature.