When using SMTP, we need to ensure either our application having issue or SMTP is the issue. For this purpose of testing, we use telnet as a simple tools to test SMTP.
For those who are using anonymous SMTP, then below is the step by step to test :
1) telnet mail.test.com 25
2) EHLO mail.test.com
3) MAIL FROM: firstname.lastname@example.org
4) RCPT TO: