++----------------+---------+-----------------+--------------+
+|keep_environment|Use: main|Type: string list|Default: unset|
++----------------+---------+-----------------+--------------+
+
+This option contains a string list of environment variables to keep. You have
+to trust these variables or you have to be sure that these variables do not
+impose any security risk. Keep in mind that during the startup phase Exim is
+running with an effective UID 0 in most installations. As the default value is
+an empty list, the default environment for using libraries, running embedded
+Perl code, or running external binaries is empty, and does not not even contain
+PATH or HOME.
+
+Actually the list is interpreted as a list of patterns (10.1), except that it
+is not expanded first.
+
+WARNING: Macro substitution is still done first, so having a macro FOO and
+having FOO_HOME in your keep_environment option may have unexpected results.
+You may work around this using a regular expression that does not match the
+macro name: ^[F]OO_HOME$.
+
+Current versions of Exim issue a warning during startupif you do not mention
+keep_environment or add_environment in your runtime configuration file.
+