TCP/IP-channel-specific options: CHECK_SOURCE (0 or 1)
The MTA'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
ident* 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
CHECK_SOURCE 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
identnonenumeric 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
CHECK_SOURCE to 0 also effectively blocks
switchchannel channel switching from taking effect.