MAX HELO DOMAIN LENGTH

TCP/IP-channel-specific options: MAX_HELO_DOMAIN_LENGTH (integer)
This SMTP/LMTP server option may be used to specify a maximum length of host name that a remote client may put on its HELO or EHLO line. If a longer name is seen on the HELO or EHLO line, then the SMTP or LMTP  server will reject the command with an error (in the case of the SMTP  server) of  501 5.5.0 Argument to HELO is too long. or 501 5.5.0 Argument to EHLO is too long. or (in the case of the LMTP server) of 501 5.5.0 Argument to LHLO is too long. as appropriate. The default is no limit.

See also:
 * TCPIP-channel-specific options