"Email verification" sounds like one thing, but it's really a stack of checks of increasing depth and cost. Knowing what each layer actually proves helps you pick the right level instead of overpaying for verification you don't need.
Layer 1: syntax
The cheapest check: does the string look like a valid email address? A pragmatic regex catches obvious garbage (asdf, a@@b, trailing spaces). It's instant and free, but weak on its own: nobody@asdf.asdf passes syntax and can't receive a single message.
Layer 2: domain and MX records
Next, does the domain actually accept mail? Every domain that receives email publishes MX (mail exchanger) records in DNS pointing to its mail servers. A quick DNS lookup tells you whether any exist. No MX (and no fallback A record) means the domain can't receive mail, so the address is undeliverable no matter how it's spelled. This single step removes a large class of fakes and dead domains.
