From time to time is really helps to be able to manually test smtp via good old telnet. Every smtp command is provided via text commands. Here is a simple connections:
telnet smtp.host.com 25
Send a hello command with your domain name
ehlo mail.myserver.com 250-linuxmonkey.localdomain 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN
Sender address
MAIL FROM: mail@domain.ext
Destination address
RCPT TO: mail@otherdomain.ext
Input your message:
DATA SUBJECT: MY SUBJECT MESSAGE STUFFF .
And you have sent a message.