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
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_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
expandable) can also be used to disable expansion on a list-by-list basis (and can be used on user entries as well).