diff --git a/configure.ac b/configure.ac
index d4a7641eb98806dad689be27dfaa90ad798c4309..8a62065e274bb8ea4766d65b881a31a72c9288e0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,3 +1,7 @@
+AC_PREREQ([2.59])
+
+# Be sure to update ABI version also if anything changes that might require
+# recompiling plugins. Most importantly that means if any structs are changed.
 AC_INIT([Pigeonhole], [0.4.11], [dovecot@dovecot.org], [dovecot-2.2-pigeonhole])
 AC_DEFINE_UNQUOTED([PIGEONHOLE_ABI_VERSION], "0.4.ABIv2($PACKAGE_VERSION)", [Pigeonhole ABI version])