SMTP Server Test
You can use this reverse SMTP server testing tool to check and verify the configuration of your SMTP mail server software.
What is an SMTP Server?
SMTP, or the Simple Mail Transfer Protocol, is a communication protocol for electronic mail transmission. First defined in 1982 by RFC 821, SMTP defines how servers on the internet that send and receive email communicate with each other. Knowing this we can deduce that an SMTP server is a special kind of computer connected to the internet running software that handles the inbound and outbound mail for a given domain name or many domain names.
A list comparing different SMTP server software can be found on this Wikipedia page, outlining the operating system(s) and features supported by a given piece of software. Some of the most common SMTP server software running on Linux/Unix based systems includes Postfix, Exim, and Sendmail.