SmtpTest
SmtpTest is a small, 64bit Windows program that lets you easily test SMTP servers and relays. It is easier than using a telnet session to manually test SMTP.
SmtpTest can automatically lookup the MX record for an email domain, or you can manually specify the SMTP host and port. It includes support for SSL and basic authentication.
SmtpTest is free for both personal and commercial use.
If you find SmtpTest useful please consider donating.