CHECK SOURCE

TCP/IP-channel-specific options: CHECK_SOURCE (0 or 1)
The MTA&#x27;s SMTP and LMTP server normally attempts to determine the name of the host from which a connection has been received, as specified by  the   channel options. When the determined name does not match the name presented by the remote SMTP/LMTP client on the HELO/EHLO/LHLO line, the    TCP/IP-channel-specific  option controls whether the name found from a DNS lookup (or the IP  domain literal, if DNS lookups have been disabled such as with the    channel option)  is included in the  constructed Received: header as a comment after the presented name. A value of 1 (the default) enables the inclusion of the determined name  when different from the presented name. A value of 0 disables the inclusion of any such comment and thus eliminates one of the more  useful checks of message validity. For the SMTP server, setting   to 0 also effectively blocks     channel switching from taking effect.

See also:
 * identnonenumeric Option
 * allowswitchchannel Option
 * TCPIP-channel-specific options