What can be checked without sending mail
Three things are verifiable from outside. Syntax: whether the address is well-formed at all. Domain: whether the part after the at sign actually resolves. MX records: whether that domain has mail servers configured to receive anything.
Those three catch the overwhelming majority of bad addresses, and nearly all of them are typos - gmail.co, hotmial.com, a missing letter in a company domain. Catching them at the point of entry is worth far more than cleaning the list later.
What cannot be checked
Whether a specific mailbox exists is not reliably knowable. Most mail servers deliberately accept every address at the SMTP level and discard the unknown ones afterwards, specifically to prevent address harvesting. Any service claiming certainty about individual mailbox existence is overstating what the protocol allows.
Whether the person actually reads that mailbox is not knowable at all. The only real confirmation is a confirmation email - which is why double opt-in remains the standard for mailing lists and why it is worth the friction.
Disposable addresses and why they matter
Disposable addresses come from services that hand out a mailbox for a few minutes. They are used to get past a signup wall without giving a real address, which means a list full of them looks larger than it is and performs far worse.
The downstream damage is bigger than the wasted sends. Messages to abandoned addresses go unread and unopened, which drags down your sender reputation and reduces deliverability to the real subscribers who did want to hear from you.
Frequently asked questions
- Can you tell if an email address actually exists?
- Not reliably. Most mail servers accept all addresses at the SMTP level to prevent harvesting, so only a confirmation email proves a mailbox is real and monitored.
- Does this send an email to the address?
- No. Nothing is sent. The checks are syntax, domain resolution and MX records, all done from outside.
- What is a disposable email address?
- A temporary mailbox from a service that provides addresses for a few minutes. They inflate list sizes, never convert, and harm sender reputation through non-engagement.
- Why does a valid-looking address fail the check?
- Usually the domain has no MX records, meaning it cannot receive mail at all. A misspelled domain is the most common cause.