TestMail system uses Standard Requests For Comments (RFC) specifications and Internet Service Providers (ISPs) email format rules to determine whether an email syntax is acceptable.

A deliverable email must have a domain name that is registered in the Domain Name System (DNS). DNS is a sort of phone or address book for the internet. DNS translates a domain name to an IP address.

A mail exchanger record (MX record) specifies the mail server responsible for accepting email messages on behalf of a domain name. Failing DNS/MX check indicates a given email address is not deliverable.

Simple Mail Transfer Protocol (SMTP) is part of the application layer of the TCP/IP protocol. A network protocol defines rules and conventions for communication between network devices. SMTP is a protocol used for transferring emails across the internet. The SMTP mailbox check connects to a given mail server via SMTP, and asks the mail server whether a particular user account is deliverable.

Disposable email addresses (DEAs) are short-lived inboxes under a temporary address. Some users may use DEAs to keep their real inbox spam-free. The Disposable Email Addresses check screens and flags known DEAs to improve email deliverability.

Realtime blacklist (RBL) check tests a given mail server IP address against known spam email blacklists from multiple sources in order to determine whether the mail server is blacklisted. If a mail server has been blacklisted, some email sent through the mail server may not be delivered. Using Email blacklists is a common way of reducing spam.

An email address may contain useful information, such as: first/last names, first/last name initials, email format rules, special numbers, etc. TestMail system takes machine learning natural language processing approach to parse a given email to extract possible useful hints.