An attempt to send email via a secured SMTP server may result in run-time errors such as

550 Relaying Denied

The SMTP protocol provides several authentication methods through which a mail client submits its security credentials to the server during an SMTP session. SMTP servers usually support at least one authentication method, but most support several.

The most common SMTP authentication methods are as follows:

1. AUTH=LOGIN

This is the simplest authentication method where the username and password are sent to the SMTP server in clear text (that is, unencrypted) although the values are Base64-encoded. Authentication parameters can therefore be intercepted.

2. CRAM-MD5

The server sends a random string to the client. Both the client and the server calculate an MD5 digest of the concatenation of the random string and the password; the client then sends the server the result of its calculation. The server compares the two results.

This method is more secure than AUTH=LOGIN because the actual password never gets sent to the server.

2. NTLM

This is Microsoft's proprietary authentication protocol also known as "Challenge/Response". It is also secure in the sense that the password never gets transmitted over the network.