To answer that question, a study used functional Magnetic Resonance Imaging (MRI) to understand differences in how online and offline media communicate a message. The research was conducted by Millward Brown in conjunction with the University of Bangor in Wales, UK.
Ten males and ten females were shown images presented on screen (to simulate the online experience of email) and then on printed cards (to simulate direct mail). To summarize the findings:
"The printed material facilitated greater emotional processing–seen as more brain activity in the areas associated with visual and spatial information–suggesting it is more real to the subject. Greater emotional involvement leads to more positive brand associations, along with easier brand recall. This more tangible experience is also more readily internalized by the viewer, which means the ads should have a more personal effect that in turn positively influences motivation to take action, like buying a product or making a donation".
In contrast, the response to online materials demonstrated a difficulty to focus.