diff -Naur cyrus-imapd-2.3.10.orig/lib/auth_unix.c cyrus-imapd-2.3.10/lib/auth_unix.c
--- cyrus-imapd-2.3.10.orig/lib/auth_unix.c	2007-09-27 22:02:45.000000000 +0200
+++ cyrus-imapd-2.3.10/lib/auth_unix.c	2007-11-01 11:36:56.000000000 +0100
@@ -48,6 +48,7 @@
 #include <stdlib.h>
 #include <pwd.h>
 #include <grp.h>
+#include <stdio.h>
 #include <ctype.h>
 #include <string.h>
 
@@ -226,9 +227,12 @@
 #ifdef HAVE_GETGROUPLIST
     gid_t gid, *groupids = NULL;
     int ret, ngroups = 0;
-#else
-    char **mem;
 #endif
+    char **mem;
+    FILE *groupcache;
+    char fnamebuf[1024];
+    const char *config_dir =
+        libcyrus_config_getstring(CYRUSOPT_CONFIG_DIR);
 
     identifier = mycanonifyid(identifier, 0);
     if (!identifier) return 0;
@@ -245,6 +249,32 @@
 
     pwd = getpwnam(identifier);
 
+/*
+ * use the groupcache file if it exists, otherwise fall back
+ * to getgrouplist()/getgrent()
+ */
+    strcpy(fnamebuf, config_dir);
+    strcat(fnamebuf, "/group.cache");
+    groupcache = fopen(fnamebuf, "r");
+  if (groupcache) {
+    setgrent();
+    while ((grp = fgetgrent(groupcache))) {
+	for (mem = grp->gr_mem; *mem; mem++) {
+	    if (!strcmp(*mem, identifier)) break;
+	}
+
+	if (*mem || (pwd && pwd->pw_gid == grp->gr_gid)) {
+	    newstate->ngroups++;
+	    newstate->group = (char **)xrealloc((char *)newstate->group,
+						newstate->ngroups * sizeof(char *));
+	    newstate->group[newstate->ngroups-1] = xstrdup(grp->gr_name);
+	}
+    }
+    endgrent();
+    fclose(groupcache);
+  }
+  else { /* begin !groupcache */
+
 #ifdef HAVE_GETGROUPLIST
     gid = pwd ? pwd->pw_gid : (gid_t) -1;
 
@@ -302,6 +332,8 @@
     endgrent();
 #endif /* HAVE_GETGROUPLIST */
 
+  } /* end !groupcache */
+
     return newstate;
 }