Email verification services typically employ several checks to ensure the validity of any email address, which include the following:
Format check: The Internet Engineering Task Force (IETF) has established standards that every email address must follow. IETF, for instance, requires that each email address have a local-part (usually a person’s or department’s name), the @ sign, and the domain as in name@company[.]com. Any email address that does not conform to these standards is usually deemed invalid by the best email verifiers. This check uses a syntactic email validation engine that determines if an email address follows IETF specifications.
Simple Mail Transfer Protocol (SMTP) check: Any email address that cannot receive messages is undoubtedly invalid. It should have a corresponding SMTP server to do that. The best email verification services check for mailbox existence by emulating email sending to check for an SMTP connection.
Domain Name System (DNS) and mail exchange (MX) record check: Any domain (including email domains) needs a corresponding DNS server to direct users to the correct pages (or, in this case, mailboxes). Determining if an email address, therefore, has appropriately configured DNS records (specifically MX records) and a corresponding DNS server (MX server in this case) is a crucial step in email validation. Any email address without a corresponding MX server and adequately configured MX records cannot receive messages.
Free check: Many if not all signup pages require users to key in their corporate instead of personal email addresses since these are more useful for lead generation in business-to-business (B2B) scenarios. The best email verifiers can check if any given email address is provided by free services like Gmail or Yahoo!
Disposable check: While not all disposable email address users are malicious (some may just take their privacy more seriously than others), some, unfortunately, are freemium abusers. The best email verification services can instantly tell users if an email address is from disposable or temporary email services like GuerillaMail, Mailinator, or 10 Minute Mail.
Catch-all check: Catch-all addresses are those that accept all emails that are sent to a domain name. Companies typically use catch-all email addresses for general inquiries that users may not know who exactly in the organizations should receive. Examples of these include info@company[.]com, inquiry@company[.]com, or support@company[.]com. These are not invalid email addresses, though. Their owners are just undefined, but for marketing purposes, they may not be ideal contact means.