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TCP/IP-channel-specific options: DISABLE_EXPAND (0 or 1)

The SMTP EXPN command is used to expand mailing lists. Exposing the contents of mailing lists to outside scrutiny may constitue a breach of security for some sites. The SMTP server option DISABLE_EXPAND, when set to 1, disables the EXPN command completely, causing the SMTP server to issue the response

550 5.7.2 EXPN command has been disabled 

to any EXPN command. The default value is 0, which causes the EXPN command to work normally.

See also the expnallow, expndefault, and expndisable channel options that can be used to control this behavior on a per-channel, rather than per-SMTP-server, basis. And see the expandable_default MTA option which sets an MTA-wide default.

Note that mailing list expansion can also be blocked on a list-by-list basis with the Unified Configuration alias options alias_expandable and alias_nonexpandable, or the [EXPANDABLE] and [NONEXPANDABLE] alias file named parameters.

Note that an LDAP attribute named by the ldap_expandable MTA option (normally set to the attributes mgmanMemberVisibility and expandable) can also be used to disable expansion on a list-by-list basis (and can be used on user entries as well).

See also: