http://dovecot.org/list/dovecot/2007-April/021429.html

diff -ru dovecot-1.0.rc27.orig/src/plugins/quota/quota-plugin.c dovecot-1.0.rc27/src/plugins/quota/quota-plugin.c
--- dovecot-1.0.rc27.orig/src/plugins/quota/quota-plugin.c	2007-03-06 17:34:47.000000000 +0100
+++ dovecot-1.0.rc27/src/plugins/quota/quota-plugin.c	2007-03-31 22:18:55.000000000 +0200
@@ -21,9 +21,24 @@
 
 	env = getenv("QUOTA");
 	if (env != NULL) {
+		struct quota_setup *setup;
+		const char *name;
+		unsigned int i;
 		quota_set = quota_init();
 		/* Currently we support only one quota setup */
-		(void)quota_setup_init(quota_set, env, TRUE);
+		setup = quota_setup_init(quota_set, env, TRUE);
+
+		name = "QUOTA_WARNING";
+		env = getenv(name);
+		i = 1;
+		t_push();
+		while (env != NULL) {
+			(void)quota_warning_init(setup, env);
+
+			name = t_strdup_printf("QUOTA_WARNING%u", ++i);
+			env = getenv(name);
+		}
+		t_pop();
 
 		quota_next_hook_mail_storage_created =
 			hook_mail_storage_created;
diff -ru dovecot-1.0.rc27.orig/src/plugins/quota/quota-private.h dovecot-1.0.rc27/src/plugins/quota/quota-private.h
--- dovecot-1.0.rc27.orig/src/plugins/quota/quota-private.h	2007-03-12 21:34:50.000000000 +0100
+++ dovecot-1.0.rc27/src/plugins/quota/quota-private.h	2007-03-17 00:06:23.000000000 +0100
@@ -22,6 +22,9 @@
 	/* List of quota roots. It's array because there shouldn't be many. */
 	array_t ARRAY_DEFINE(roots, struct quota_root *);
 
+	/* List of quota warnings. There should probably be few. */
+	array_t ARRAY_DEFINE(warnings, struct quota_warning *);
+
 	unsigned int user_root:1;
 };
 
@@ -86,6 +89,21 @@
 	unsigned int idx;
 };
 
+enum quota_warning_limit_kind {
+	QUOTA_WARNING_NO_LIMIT,
+	QUOTA_WARNING_PERCENT_LIMIT,
+	QUOTA_WARNING_ABSOLUTE_LIMIT
+};
+
+struct quota_warning {
+	struct quota_setup *setup;
+	uint64_t storage_limit;
+	unsigned int count_limit;
+	enum quota_warning_limit_kind storage_limit_kind:2;
+	enum quota_warning_limit_kind count_limit_kind:2;
+	char *command;
+};
+
 struct quota_transaction_context {
 	array_t ARRAY_DEFINE(root_transactions,
 			     struct quota_root_transaction_context *);
diff -ru dovecot-1.0.rc27.orig/src/plugins/quota/quota.c dovecot-1.0.rc27/src/plugins/quota/quota.c
--- dovecot-1.0.rc27.orig/src/plugins/quota/quota.c	2007-03-06 17:34:47.000000000 +0100
+++ dovecot-1.0.rc27/src/plugins/quota/quota.c	2007-03-31 22:35:22.000000000 +0200
@@ -6,6 +6,8 @@
 #include "quota-private.h"
 #include "quota-fs.h"
 
+#include <stdlib.h>
+
 unsigned int quota_module_id = 0;
 
 extern struct quota_backend quota_backend_dict;
@@ -59,6 +61,7 @@
 	setup->data = i_strdup(data);
 	setup->user_root = user_root;
 	ARRAY_CREATE(&setup->roots, default_pool, struct quota_root *, 4);
+	ARRAY_CREATE(&setup->warnings, default_pool, struct quota_warning *, 4);
 
 	t_push();
 	p = strchr(setup->data, ':');
@@ -107,6 +110,16 @@
 	}
 
 	array_free(&setup->roots);
+
+	while (array_count(&setup->warnings) > 0) {
+		struct quota_warning *const *warning;
+
+		warning = array_idx(&setup->warnings, 0);
+		quota_warning_deinit(*warning);
+	}
+
+	array_free(&setup->warnings);
+
 	i_free(setup->data);
 	i_free(setup);
 }
@@ -157,6 +170,82 @@
 	array_free(&module_contexts);
 }
 
+struct quota_warning *
+quota_warning_init(struct quota_setup *setup, const char *data)
+{
+	const char *p;
+	char *q;
+	const char *const *args;
+	unsigned long long val;
+	struct quota_warning *warning;
+
+	warning = i_new(struct quota_warning, 1);
+	warning->setup = setup;
+
+	p = strchr(data, ' ');
+	if (p == NULL)
+		i_fatal("quota warning: No command specified: %s", data);
+	warning->command = i_strdup(p+1);
+
+	t_push();
+
+	args = t_strsplit(t_strdup_until(data, p), ":");
+	for (; *args != '\0'; args++) {
+		if (strncmp(*args, "storage=", 8) == 0) {
+			val = strtoull(*args + 8, &q, 10);
+			if (q && (strcmp(q, "%") == 0)) {
+				warning->storage_limit = val;
+				warning->storage_limit_kind =
+					QUOTA_WARNING_PERCENT_LIMIT;
+			} else if (q && (strcmp(q, "k") == 0)) {
+				warning->storage_limit = val * 1024;
+				warning->storage_limit_kind =
+					QUOTA_WARNING_ABSOLUTE_LIMIT;
+			} else
+				i_error("quota warning: Malformed setting: %s", *args);
+		} else if (strncmp(*args, "messages=", 9) == 0) {
+			val = strtoull(*args + 8, &q, 10);
+			if (q && (strcmp(q, "%") == 0)) {
+				warning->count_limit = val;
+				warning->count_limit_kind =
+					QUOTA_WARNING_PERCENT_LIMIT;
+			} else if (q && (strcmp(q, "") == 0)) {
+				warning->count_limit = val;
+				warning->count_limit_kind =
+					QUOTA_WARNING_ABSOLUTE_LIMIT;
+			} else
+				i_error("quota warning: Malformed setting: %s", *args);
+		} else {
+			i_error("quota warning: Unknown setting: %s", *args);
+		}
+	}
+
+	t_pop();
+
+	array_append(&setup->warnings, &warning, 1);
+
+	return warning;
+}
+
+void quota_warning_deinit(struct quota_warning *warning)
+{
+	struct quota_warning *const *warnings;
+	unsigned int i, count;
+
+	/* remove from setup */
+	warnings = array_get(&warning->setup->warnings, &count);
+	for (i = 0; i < count; i++) {
+		if (warnings[i] == warning) {
+			array_delete(&warning->setup->warnings, i, 1);
+			break;
+		}
+	}
+	i_assert(i != count);
+
+	i_free(warning->command);
+	i_free(warning);
+}
+
 void quota_add_user_storage(struct quota *quota, struct mail_storage *storage)
 {
 	struct quota_setup *const *setups;
@@ -388,15 +477,62 @@
 	i_free(ctx);
 }
 
+#define CHECK_PERCENT(val, cur, diff, limit) \
+	(100 * ((limit) - (cur) - (diff)) > (val) * (limit))
+#define CHECK_ABSOLUTE(val, cur, diff, limit) \
+	((limit) - (cur) - (diff) > (val))
+
+#define CHECK2_PERCENT(val, cur, diff, size, limit) \
+	(CHECK_PERCENT(val, cur, diff, limit) \
+	 && !CHECK_PERCENT(val, cur, (diff)+(size), limit))
+#define CHECK2_ABSOLUTE(val, cur, diff, size, limit) \
+	(CHECK_ABSOLUTE(val, cur, diff, limit) \
+	 && !CHECK_ABSOLUTE(val, cur, (diff)+(size), limit))
+
+#define CHECK(kind, val, cur, diff, size, limit) \
+	((((kind) == QUOTA_WARNING_PERCENT_LIMIT) \
+	  && (CHECK2_PERCENT(val, cur, diff, size, limit))) \
+	 || (((kind) == QUOTA_WARNING_ABSOLUTE_LIMIT) \
+	     && (CHECK2_ABSOLUTE(val, cur, diff, size, limit))))
+
 int quota_default_try_alloc_bytes(struct quota_root_transaction_context *ctx,
 				  uoff_t size, bool *too_large_r)
 {
+	struct quota_warning *const * warnings;
+	unsigned int i, count;
 	int ret;
 
 	ret = quota_default_test_alloc_bytes(ctx, size, too_large_r);
 	if (ret <= 0 || ctx->disabled)
 		return ret;
 
+	warnings = array_get(&ctx->root->setup->warnings, &count);
+
+	for (i = 0; i < count; i++) {
+		struct quota_warning *warning = warnings[i];
+		bool run_command = FALSE;
+
+		if (CHECK(warning->storage_limit_kind,
+			  warning->storage_limit,
+			  ctx->bytes_current,
+			  ctx->bytes_diff,
+			  size,
+			  ctx->bytes_limit))
+			run_command = TRUE;
+
+		if (CHECK(warning->count_limit_kind,
+			  warning->count_limit,
+			  ctx->count_current,
+			  ctx->count_diff,
+			  1,
+			  ctx->count_limit))
+			run_command = TRUE;
+
+		if (run_command)
+			system(warning->command);
+
+	}
+
 	ctx->count_diff++;
 	ctx->bytes_diff += size;
 	return 1;
diff -ru dovecot-1.0.rc27.orig/src/plugins/quota/quota.h dovecot-1.0.rc27/src/plugins/quota/quota.h
--- dovecot-1.0.rc27.orig/src/plugins/quota/quota.h	2007-03-06 17:34:47.000000000 +0100
+++ dovecot-1.0.rc27/src/plugins/quota/quota.h	2007-03-16 23:21:33.000000000 +0100
@@ -31,6 +31,10 @@
 quota_root_init(struct quota_setup *setup, const char *name);
 void quota_root_deinit(struct quota_root *root);
 
+struct quota_warning *
+quota_warning_init(struct quota_setup *setup, const char *data);
+void quota_warning_deinit(struct quota_warning *warning);
+
 /* List all quota roots. Returned quota roots are freed by quota_deinit(). */
 struct quota_root_iter *quota_root_iter_init(struct mailbox *box);
 struct quota_root *quota_root_iter_next(struct quota_root_iter *iter);