diff --git a/src/lib-smtp/smtp-server-cmd-data.c b/src/lib-smtp/smtp-server-cmd-data.c
index 674416609c..122aa5a2d9 100644
--- a/src/lib-smtp/smtp-server-cmd-data.c
+++ b/src/lib-smtp/smtp-server-cmd-data.c
@@ -21,6 +21,33 @@ struct cmd_data_context {
 	bool chunk_last:1;
 };
 
+static void
+smtp_server_cmd_data_size_limit_exceeded(struct smtp_server_cmd_ctx *cmd)
+{
+	struct smtp_server_command *command = cmd->cmd;
+
+	smtp_server_command_fail(command, 552, "5.2.3",
+				 "Message size exceeds administrative limit");
+}
+
+bool smtp_server_cmd_data_check_size(struct smtp_server_cmd_ctx *cmd)
+{
+	struct smtp_server_connection *conn = cmd->conn;
+	const struct smtp_server_settings *set = &conn->set;
+
+	i_assert(conn->state.state == SMTP_SERVER_STATE_DATA);
+
+	if (conn->state.data_input == NULL)
+		return TRUE;
+	if (set->max_message_size == 0)
+		return TRUE;
+	if (conn->state.data_input->v_offset <= set->max_message_size)
+		return TRUE;
+
+	smtp_server_cmd_data_size_limit_exceeded(cmd);
+	return FALSE;
+}
+
 bool smtp_server_connection_data_check_state(struct smtp_server_cmd_ctx *cmd)
 {
 	struct smtp_server_connection *conn = cmd->conn;
@@ -213,6 +240,9 @@ static int cmd_data_handle_input(struct smtp_server_cmd_ctx *cmd)
 
 	i_assert(data_cmd != NULL);
 
+	if (!smtp_server_cmd_data_check_size(cmd))
+		return -1;
+
 	/* continue reading from client */
 	smtp_server_command_ref(command);
 	i_assert(callbacks != NULL &&
@@ -220,7 +250,10 @@ static int cmd_data_handle_input(struct smtp_server_cmd_ctx *cmd)
 	ret = callbacks->conn_cmd_data_continue(conn->context,
 		cmd, conn->state.trans);
 	if (ret >= 0) {
-		if (!i_stream_have_bytes_left(conn->state.data_input)) {
+		if (!smtp_server_cmd_data_check_size(cmd)) {
+			smtp_server_command_unref(&command);
+			return -1;
+		} else if (!i_stream_have_bytes_left(conn->state.data_input)) {
 			smtp_server_command_debug(cmd,
 				"End of data");
 			smtp_server_command_input_lock(cmd);
@@ -451,16 +484,27 @@ int smtp_server_connection_data_chunk_add(struct smtp_server_cmd_ctx *cmd,
 	bool client_input)
 {
 	struct smtp_server_connection *conn = cmd->conn;
+	const struct smtp_server_settings *set = &conn->set;
 	struct smtp_server_command *command = cmd->cmd;
 	struct cmd_data_context *data_cmd =
 		(struct cmd_data_context *)command->data;
 	struct istream *input;
+	uoff_t new_size;
 
 	i_assert(data_cmd != NULL);
 
 	if (!smtp_server_connection_data_check_state(cmd))
 		return -1;
 
+	/* check message size increase early */
+	new_size = conn->state.data_size + chunk_size;
+	if (new_size < conn->state.data_size ||
+	    (set->max_message_size > 0 && new_size > set->max_message_size)) {
+		smtp_server_cmd_data_size_limit_exceeded(cmd);
+		return -1;
+	}
+	conn->state.data_size = new_size;
+
 	command->hook_replied = (chunk_last ?
 		cmd_data_replied : cmd_data_chunk_replied);
 
diff --git a/src/lib-smtp/smtp-server-cmd-helo.c b/src/lib-smtp/smtp-server-cmd-helo.c
index 2aed40e100..6657dfb047 100644
--- a/src/lib-smtp/smtp-server-cmd-helo.c
+++ b/src/lib-smtp/smtp-server-cmd-helo.c
@@ -122,6 +122,15 @@ smtp_server_cmd_helo_run(struct smtp_server_cmd_ctx *cmd, const char *params,
 					"ENHANCEDSTATUSCODES");
 			}
 			smtp_server_reply_ehlo_add(reply, "PIPELINING");
+			if ((caps & SMTP_CAPABILITY_SIZE) != 0) {
+				uoff_t cap_size = conn->set.max_message_size;
+				if (cap_size > 0 && cap_size != (uoff_t)-1) {
+					smtp_server_reply_ehlo_add_param(reply,
+						"SIZE", "%"PRIuUOFF_T, cap_size);
+				} else {
+					smtp_server_reply_ehlo_add(reply, "SIZE");
+				}
+			}
 			if ((caps & SMTP_CAPABILITY_STARTTLS) != 0)
 				smtp_server_reply_ehlo_add(reply, "STARTTLS");
 			smtp_server_reply_ehlo_add(reply, "VRFY");
diff --git a/src/lib-smtp/smtp-server-cmd-mail.c b/src/lib-smtp/smtp-server-cmd-mail.c
index a270f5b5df..5c76709fc3 100644
--- a/src/lib-smtp/smtp-server-cmd-mail.c
+++ b/src/lib-smtp/smtp-server-cmd-mail.c
@@ -151,6 +151,13 @@ void smtp_server_cmd_mail(struct smtp_server_cmd_ctx *cmd,
 		return;
 	}
 
+	if ((caps & SMTP_CAPABILITY_SIZE) != 0 && set->max_message_size > 0 &&
+	    mail_data->params.size > set->max_message_size) {
+		smtp_server_reply(cmd, 552, "5.2.3",
+			"Message size exceeds administrative limit");
+		return;
+	}
+
 	mail_data->path = smtp_address_clone(cmd->pool, path);
 	mail_data->timestamp = ioloop_timeval;
 
diff --git a/src/lib-smtp/smtp-server-connection.c b/src/lib-smtp/smtp-server-connection.c
index 1c0fdb33d4..e667bf305a 100644
--- a/src/lib-smtp/smtp-server-connection.c
+++ b/src/lib-smtp/smtp-server-connection.c
@@ -812,6 +812,24 @@ smtp_server_connection_alloc(struct smtp_server *server,
 		smtp_command_limits_merge(&conn->set.command_limits,
 					  &set->command_limits);
 
+		conn->set.max_message_size = set->max_message_size;
+		if (set->max_message_size == 0 ||
+		    set->max_message_size == (uoff_t)-1) {
+			conn->set.command_limits.max_data_size = UOFF_T_MAX;
+		} else if (conn->set.command_limits.max_data_size != 0) {
+			/* explicit limit given */
+		} else if (set->max_message_size >
+			(UOFF_T_MAX - SMTP_SERVER_DEFAULT_MAX_SIZE_EXCESS_LIMIT)) {
+			/* very high limit */
+			conn->set.command_limits.max_data_size = UOFF_T_MAX;
+		} else {
+			/* absolute maximum before connection is closed in DATA
+			   command */
+			conn->set.command_limits.max_data_size =
+				set->max_message_size +
+					SMTP_SERVER_DEFAULT_MAX_SIZE_EXCESS_LIMIT;
+		}
+
 		if (set->xclient_extensions != NULL) {
 			server->set.xclient_extensions =
 				p_strarray_dup(pool, set->xclient_extensions);
diff --git a/src/lib-smtp/smtp-server-private.h b/src/lib-smtp/smtp-server-private.h
index dc843aaf41..58972646ca 100644
--- a/src/lib-smtp/smtp-server-private.h
+++ b/src/lib-smtp/smtp-server-private.h
@@ -5,10 +5,11 @@
 
 #include "smtp-server.h"
 
-#define SMTP_SERVER_COMMAND_POOL_MAX          (8 * 1024)
+#define SMTP_SERVER_COMMAND_POOL_MAX              (8 * 1024)
 
-#define SMTP_SERVER_DEFAULT_MAX_COMMAND_LINE  (4 * 1024)
-#define SMTP_SERVER_DEFAULT_MAX_BAD_COMMANDS  10
+#define SMTP_SERVER_DEFAULT_MAX_COMMAND_LINE      (4 * 1024)
+#define SMTP_SERVER_DEFAULT_MAX_BAD_COMMANDS      10
+#define SMTP_SERVER_DEFAULT_MAX_SIZE_EXCESS_LIMIT (1024*1024)
 
 #define SMTP_SERVER_DEFAULT_CAPABILITIES \
 	(SMTP_CAPABILITY_SIZE | SMTP_CAPABILITY_ENHANCEDSTATUSCODES | \
@@ -110,6 +111,7 @@ struct smtp_server_state_data {
 	struct istream *data_input, *data_chain_input;
 	struct istream_chain *data_chain;
 	unsigned int data_chunks;
+	uoff_t data_size;
 
 	bool data_failed:1;
 };
diff --git a/src/lib-smtp/smtp-server.c b/src/lib-smtp/smtp-server.c
index 54ada3700f..4cc0fabef8 100644
--- a/src/lib-smtp/smtp-server.c
+++ b/src/lib-smtp/smtp-server.c
@@ -51,6 +51,7 @@ struct smtp_server *smtp_server_init(const struct smtp_server_settings *set)
 		set->max_bad_commands : SMTP_SERVER_DEFAULT_MAX_BAD_COMMANDS);
 	server->set.max_recipients = set->max_recipients;
 	server->set.command_limits = set->command_limits;
+	server->set.max_message_size = set->max_message_size;
 
 	if (set->xclient_extensions != NULL) {
 		server->set.xclient_extensions =
diff --git a/src/lib-smtp/smtp-server.h b/src/lib-smtp/smtp-server.h
index bc749b06f2..972d5219c5 100644
--- a/src/lib-smtp/smtp-server.h
+++ b/src/lib-smtp/smtp-server.h
@@ -258,6 +258,9 @@ struct smtp_server_settings {
 	/* command limits */
 	struct smtp_command_limits command_limits;
 
+	/* message size limit */
+	uoff_t max_message_size;
+
 	/* accept these additional custom XCLIENT fields */
 	const char *const *xclient_extensions;
 
@@ -454,6 +457,10 @@ void smtp_server_cmd_auth_success(struct smtp_server_cmd_ctx *cmd,
 	const char *username, const char *success_msg)
 	ATTR_NULL(3);
 
+/* DATA */
+
+bool smtp_server_cmd_data_check_size(struct smtp_server_cmd_ctx *cmd);
+
 /*
  * Reply
  */