Symantec™ Security Gateways Reference Guide - Sawmill

Understanding accessProxies59Extended SMTPThe SMTP proxy also supports extended SMTP (ESMTP) commands, by this support must be enabled.Defined in RFC 2821, ESMTP is a set of extensions to SMTP. Table 4-5 lists the SMTP extensions theSymantec SMTP proxy supports.Table 4-5CommandEHLOESMTPAUTHATRNETRNEXPNVRFYSupported ESMTP commandsDescriptionIdentifies the SMTP client to the SMTP server. The argument field contains the fully-qualified domainname, if one is available. This command announces that the SMTP client supports the Extended SMTPcommand set, and is inquiring if the SMTP server does the same.Provides access to the Extended Simple Mail Transfer Protocol. ESMTP is enabled by default. When it isenabled, the other service extensions on this tab are enabled.Lets the client send user name and password to authenticate with the server. Authentication is enabledby default.Permits an on-demand mail relay from the server to the client by turning the existing connectionaround. ATRN is disabled by default.Note that the security gateway does not support authentication mechanisms that result in theconnection being encrypted.Lets the client access mail. In this case, the server is requested to initiate a separate connection to theclient for the purpose of mail relay from the server to the client. ETRN is disabled by default.Allows for the expansion of mailing lists. EXPN is disabled by default. Enabling this option exposesinformation about your internal network to untrusted sites and is therefore not recommended.Allows verification of mail addresses. VRFY is disabled by default. Enabling this option exposesinformation about your internal network to untrusted sites and is therefore not recommended.SMTP proxy communicationWhen interacting with your mail exchange server, whether it be connecting, performing various mailrelated functions, or closing a connection, the server responds with a code. These codes tell other mailservers and mail clients how to behave. Table 4-6 provides a brief description of the codes that might bereturned by an SMTP server.Table 4-6CodeDescriptionSMTP return codes211 A system status message.214 A help message for a human reader follows.220 Service ready.221 Service closing.250 Requested action taken and completed.251 The recipient is not local to the server, but the server accepts and forwards the message.252 The recipient cannot be verified, but the server accepts the message and attempts delivery.354 Start message input and end with ".". This indicates that the server is ready to accept the message itself.421 Service is not available and the connection will be closed.450 The requested command failed because the user’s mailbox was unavailable.451 The command was aborted due to server error.