What is the “To” field?
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You know how when you write a letter, the envelope has the recipient's address but the letter inside also has "Dear So-and-So" at the top? Email works the same way. The To field is what the reader sees when they open the message. It's the "Dear So-and-So" part.
But here's the weird part: that visible To field isn't what actually controls where the email goes. Actual delivery happens through invisible SMTP commands called RCPT TO (recipient to). When you hit send, your mail server uses those commands to tell the receiving server where to deliver the message. The To field? That's just the label printed at the top of the letter for humans to read.
Most of the time, the To field and the RCPT TO commands match. You put captain@deepcurrent.io in the To field, the server sends it to captain@deepcurrent.io, and everyone's happy. But they can be different. That's how BCC works. The visible To field shows one address, but the RCPT TO commands secretly include others.
If you've ever received an email addressed to "Undisclosed Recipients," that's what happens when someone BCCs everyone and leaves the To field empty. Most mail clients won't let you send a message with no visible recipient, so they fill it in with a placeholder. It's purely cosmetic. The actual recipients are still hidden in those RCPT TO commands.
Why does this matter? Because bounces happen at the RCPT TO level, not the To field level. If your To field says lighthouse@harborpost.net but the RCPT TO command sends it somewhere else, and that somewhere else bounces, you'll get a bounce for an address you never saw in the To field. Confusing, but that's how the plumbing works.
One more practical thing: spam filters look at both. If the To field shows your address but you're actually a BCC recipient (meaning your address wasn't in the To or CC fields), that's a yellow flag. Not a guaranteed spam placement, but it's one signal among many. Legitimate bulk email to BCC lists happens all the time, but so does phishing. Filters notice.
If you're curious how all this fits into the bigger picture of how email actually moves from sender to inbox, SMTP is where to start. That's the protocol that handles the invisible commands underneath everything you see.
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