How to collect email address of site visitors

How do I collect the email addresses of ALL visitors to my web site? I’m not talking about forms to subscribe, etc. I’m talking about a script that can automatically capture the email address of any and all visitors. There must be a way, because I often get email from sites I visit even though I didn’t enter ANY information at that site. I’m not interested in spamming visitors, I simply want to send them a one time message thanking them for visiting and inviting them to return.

Thanks

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