What should I use to check mailbox existence before emailing?

If you mean “can I verify that an email address/mailbox exists before sending?”, the short answer is:

Don’t rely on it. There’s no fully reliable, universally supported way to check mailbox existence without sending an email.

Best practical options

1. Use double opt-in / verification email

   • Send a confirmation email with a link/code.
   • This is the most reliable and compliant approach.

2. Validate the address format

   • Check syntax only:
     • name@example.com is valid format
     • name@ or name@com is not
   • This catches obvious typos, not real existence.

3. Check domain/MX records

   • Verify the domain can receive email.
   • Example: example.com has MX records.
   • This tells you the domain accepts mail, not that the specific mailbox exists.

4. Use an email verification service

   • Services like ZeroBounce, NeverBounce, Kickbox, BriteVerify, etc.
   • They combine syntax, domain, MX, reputation, and other signals.
   • Still not 100% certain, but useful for list hygiene.

What not to do

• SMTP “RCPT TO” probing to see if the server accepts the recipient:
  • Often blocked, misleading, or considered abusive.
  • Many servers always accept then bounce later.
  • Can trigger rate limits or blacklisting.

Best recommendation

• For user signups or contacts: send a confirmation email
• For list cleaning: use an email verification service
• For basic pre-checks: syntax + MX lookup

If you want, I can show you how to do a quick MX check in Python, JavaScript, or shell.