Open up Telnet
telnet smtp.yourdomain.com 25

If you need SSL do this instead
openssl s_client -connect smtp.yourdomain.com:465

Tell the server where you are coming from.
ehlo localhost

Login to the server. You will need your username and password converted to Base 64.

Base 64 Encode

auth login
334 VXNl543jfWU6
username
334 UF64c3dvtmQ6
password

Now compose your email.

mail from: <your_email@yourdomain.com>
rcpt to: <destination@domain.com>
data
Type in your email 
.