diff --git a/data/postsrsd.conf.in b/data/postsrsd.conf.in
index 6d764db..ccffc2b 100644
--- a/data/postsrsd.conf.in
+++ b/data/postsrsd.conf.in
@@ -17,7 +17,8 @@ domains = {}
 # Instead of listing your local domains directly, you can also write them to a
 # file and have PostSRSd read it. This is particularly useful if you have a
 # large number of domains for which you need to act as mail forwarder. PostSRSd
-# reads this file before it chroots and drops root privileges.
+# reads this file before it chroots and drops root privileges. The file format
+# is one domain per line.
 #
 # Example:
 #     domains-file = "@POSTSRSD_CONFIGDIR@/@PROJECT_NAME@.domains"
diff --git a/doc/postsrsd.conf b/doc/postsrsd.conf
index a60a33e..969089b 100644
--- a/doc/postsrsd.conf
+++ b/doc/postsrsd.conf
@@ -17,7 +17,8 @@ domains = {}
 # Instead of listing your local domains directly, you can also write them to a
 # file and have PostSRSd read it. This is particularly useful if you have a
 # large number of domains for which you need to act as mail forwarder. PostSRSd
-# reads this file before it chroots and drops root privileges.
+# reads this file before it chroots and drops root privileges. The file format
+# is one domain per line.
 #
 # Example:
 #     domains-file = "/usr/local/etc/postsrsd.domains"