You can manually test SMTP traffic by telnetting to a remote mail server. Below are the steps:
telnet mail.remotedomain.com 25
helo mydomain.com main from: firstname.lastname@example.org rcpt to: email@example.com data subject: Test Test Message .
You should get a 220 response connecting, a 250 response on all commands up to data, a 300-level response on the data command, and then a 250 response on closing the data section with a single period.