When testing e-mail server one of the tools that are proven to be useful is plain old telnet.
Actually this is not a surprise since SMTP protocol function is similar.
Here is how to use it:
1. Start from command prompt
telnet mailhost 25
Note: Replace mailhost with your email server.
Note1: depending on the server HELO have to be replaced with EHLO
Note2: Replace server.com with your domain
Note: Again replace firstname.lastname@example.org with your e-mail address
RCPT TO: email@example.com
Note: Replace firstname.lastname@example.org with recipient e-mail address
after that optionally
SUBJECT: Your subject
then type your message
to finish place dot at new line.
6. To exit type