What are invalid addresses?
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An invalid email address is one that either breaks the structural rules defined in RFC 5322 or points to a mailbox that doesn't actually exist. If you send to it, it'll bounce back as undeliverable.
Invalid addresses come in two flavors: syntax problems and non-existent destinations.
Syntax problems are addresses that look wrong on their face. Missing the @ sign (like "userexample.com"). Double periods in the domain ("user@example..com"). Spaces where they shouldn't be ("user @example.com"). Mismatched quotes or brackets. These violate the formatting rules and most mail servers will reject them immediately.
Non-existent destinations look structurally fine but don't actually work. The domain doesn't exist ("user@thisisnotarealdomain123.com"). The domain exists but has no MX records set up, so it can't receive mail. Or the domain is real but the specific mailbox doesn't exist ("captain@gmail.com" sent to a Gmail account that was never created). When you try to send to these, the receiving server returns an NXDOMAIN error or a "mailbox not found" message, which triggers a hard bounce.
And why this matters: sending to invalid addresses is the fastest way to wreck your sender reputation. Mailbox providers like Gmail, Outlook, and Yahoo Mail expect your hard bounce rate to stay below 0.5%. Go above that and they start assuming you're buying lists, scraping addresses, or just not paying attention. Your delivery rate tanks across the board, even for valid subscribers.
How invalid addresses sneak into your list: typos at signup ("user@gmial.com" instead of "gmail.com"). Fake addresses from bot signups or people who don't want to give you their real email ("test@test.com"). Old addresses from domains that shut down. Addresses that worked two years ago but the person changed jobs or closed the account.
What to do about it: validate addresses at the point of signup with double opt-in. Run your list through a validator like Review My Emails before any big send (we'll flag syntax errors, non-existent domains, and role addresses that look sketchy). Monitor your bounce reports and remove addresses that hard bounce. If someone's address bounces, don't keep trying. That address is dead, and every attempt makes you look worse.
So worth reading next: what makes an address structurally valid and how hard bounces actually work.
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