If you used the Apple Professional Training Series 'Aperture 3' book, then this was in lesson #1 which had you set the 'Image Export' preset for the 'Email Medium - JPEG' to 25% of Original Size and to add a watermark (which happens to be the PSD file that overlays the image with 'For Review Only').
You can fix this by going to 'Aperture menu > Presets > Image Export...' and selecting the 'Email Medium - JPEG' preset and then deselecting the 'Show Watermark' option.
Note - you can also reset the 'Percentage of Original' field to %100 to get back to the defaults.
If you did not use the APTS - Aperture 3 book mentioned above, then I am not sure where you got the watermark. 😉