How can Marketo tell if a person I have emailed has been on my website?

[Anonymous]

I'm just curious how this works. Is it using the IP address of the computer that opened the email? If so, isn't that  a fairly innacurate way to match a website visitor to a known lead? What if the person opened the email while using public wifi?

Or does it work some other way?

[Edward_Masson]

with cookies dropped on the machine when they click a marketo email link or filled out a marketo form