Expnallow, expndefault, expndisable Channel Options

Responding to SMTP EXPN commands
New in MS 6.1-0.01: These options control, at a channel level, the SMTP server&#x27;s response when a sending SMTP client issues an SMTP EXPN command. (These channel options do not apply to or affect LMTP servers.) When placed on a source channel, the  option tells the SMTP server to issue a detailed, informative response. The  tells the SMTP server to provide a detailed, informative response, unless the TCP/IP channel option   has been specified. The  channel option tells the SMTP server to reject the command with an error: 550 5.7.2 EXPN command has been disabled Thus these options allow per-channel control of EXPN responses, as opposed to the  TCPIP-channel-specific option setting which normally applies to all incoming TCP/IP channels handled via the same SMTP server.

Note that even when EXPN responses are allowed in general, mailing lists or address groups may be configured to disallow EXPN expansion of their membership set of addresses. Such mailing list controls are configured via either the   alias option (in legacy configuration, the aliases file &#x5b;NONEXPANDABLE&#x5d; named parameter), or the LDAP attributes named by the    MTA option (normally the    and   LDAP attributes). The channel restriction on EXPN, if any, is applied first; only if the channel permits EXPN does any mailing list specific restriction get checked.

See also:
 * ldap_expandable MTA Option
 * alias_expandable Option
 * alias_nonexpandable Option
 * DISABLE_EXPAND
 * expandable_default MTA Option
 * SMTP and LMTP protocol channel options
 * Channel options