interesting pattern

I have noticed this lately: When someone sends mail to user@thisdomain.com , shortly thereafter a query comes from the ISP that runs the outgoing SMTP and loads http://www.thisdomain.com. The query does not load any images, just the base html page. It comes from IP usually in the same cloud as outgoing SMTP. Sometimes this happens only after the first mail from that ISP to thisdomain.com, sometimes every time. Is this anti-spam (why querying the recipient?), or some data harvesting and connecting the dots?