There are lots of articles online that will teach you "how to connect to a mail server using telnet and send an email." Lots of them have good information on SMTP command syntax. The problems start when you are trying to do something that is hard or can't be done using telnet. Because of this I wrote this handy telnet replacement tool for debugging SMTP. Below are the top reasons to use this instead of telnet.
- Authentication — Sending encoded strings using telnet requires you to build the authentication strings yourself using a MIME tool. This tool has authentication built in, so you can just specify the username and password.
- TLS Encryption — This tool can connect using SSL directly or issue the StartTLS command and work securely.
- Telnet protection — Many servers do not allow commands to be sent one character at a time. When they detect this they'll disconnect you, thinking you are a hacker. This tool buffers the response while you type and sends the full command when finished.
- Multihomed IP address selection and binding — Telnet will always use the main IP on the machine. With this tool you can choose from any IP address on the local machine, which makes troubleshooting IP address reputation issues very easy — e.g. if a single IP is blacklisted you can easily test using that IP.
- Remembering SMTP commands — This tool has macros built in for MAIL FROM, RCPT TO, DATA, and many other commands, so you don't need RFC 2821 with you while testing.
The SMTP Server Connection Diagnostics Tool, released by SocketLabs, Inc., can be found here: http://www.socketlabs.com/smtp-server-connection-diagnostics-tool/