Server name and Server port: These fields identify the domain name and the port used by the SMTP server to be queried.

Bind address: Application-level Gateway uses this IP address as the source address of the connection when connecting the SMTP server to be queried.

Cache timeout: The result of a query is stored for the period set as Cache timeout in seconds. If a new e-mail is sent to the same recipient within this interval, the stored verification result is used.

Force delivery attempt and Sender address: Send an e-mail to the recipient as if it were sent by Sender address. If the destination mail server accepts this mail, the original e-mail is sent. This option is required because many mail server implementations do not support the VRFY SMTP command properly, or it is not enabled. (The default value for Sender address is <>, which refers to the mailer daemon and is recognized by virtually all servers.)