Alias_expandable and alias_nonexpandable alias options
alias_expandable alias option (analogue of the legacy configuration alias file named parameter [EXPANDABLE]) is used to specify that the associated list can be expanded (and hence its contents seen) by various protocols which may attempt such an operation. It does not mean, or imply, that the membership of the list will be expanded into message headers; for that, instead see
alias_nonexpandable alias option (analogue of the legacy configuration alias file named parameter [NONEXPANDABLE]) specifies that the associated list may not be expanded.
alias_expandableis the default, unless the
expandable_default MTA option has been set, in which case the default is
alias_nonexpandable is useful in blocking the expansion of mailing lists via SMTP's EXPN command. Note that mailing list access controls, e.g.,
alias_auth_mapping, etc., also affect the expansion of mailing lists via SMTP's EXPN command; the SMTP server will only permit the EXPN if the SMTP client passes the access control (e.g., has issued a prior MAIL FROM: command that passes the access control).