What types of checks do validation services perform (syntax, MX, SMTP)?
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Email validation isn't a single check. It's a series of layers, each catching different problems. Think of it as a progression: Does this address look right? Does the domain exist? Is anyone actually there?
Syntax validation is the first gate. It confirms the address follows the rules. An email has to have a local part (before the @), then @, then a domain. It can't have spaces, double dots, or other illegal characters. Most addresses pass this. It's the most basic check.
Domain and MX lookups come next. The validator checks if the domain exists by looking up DNS records. More specifically, it checks for MX (Mail Exchange) records. Those records tell the world which mail servers handle email for that domain. If a domain has no MX records, email can't be delivered there. No address at that domain is valid. This catches typos in domain names (like 'gmial.com' instead of 'gmail.com').
SMTP handshake is where it gets real. The validator connects to the receiving mail server and starts a conversation like your email client would. It says 'Hello, I'm trying to send mail to [address].' The server responds with 'That address exists' or 'That address doesn't exist' (or something in between). This is the deepest check available without actually sending mail. It catches addresses that look perfect but don't actually exist.
Bonus checks that some services add: Role detection (is this info@, support@, sales@?). Disposable domain detection (is this a temporary email service like tempmail.com?). Historical activity scoring (we've seen emails from this domain fail before, so maybe be cautious).
Why this matters. Each layer removes a different type of junk from your list. Syntax catches garbage input. MX catches typos. SMTP catches fake addresses. If you skip SMTP, you're leaving hundreds of non-existent addresses on your list that will bounce later. If you skip MX, typos slip through. You want all three working together.
Before choosing a validator, ask them which checks they actually perform. Some cheap services do syntax only. Others do syntax plus MX but skip SMTP because it's slower. You get what you pay for, so know what you're buying.
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