diff --git a/src/common.h b/src/common.h
index 06c2e3e..ed278b4 100644
--- a/src/common.h
+++ b/src/common.h
@@ -76,6 +76,8 @@ extern int Pid;
 extern char Hostname[256];
 extern const char *Home;
 
+extern int BufferLimit;
+
 /* util.c */
 
 void ATTR_PRINTFLIKE(1, 2) debug( const char *, ... );
diff --git a/src/config.c b/src/config.c
index 1c181ce..20f09d9 100644
--- a/src/config.c
+++ b/src/config.c
@@ -482,6 +482,14 @@ load_config( const char *where, int pseudo )
 				}
 			}
 		}
+		else if (!strcasecmp( "BufferLimit", cfile.cmd ))
+		{
+			BufferLimit = parse_size( &cfile );
+			if (BufferLimit <= 0) {
+				error( "%s:%d: BufferLimit must be positive\n", cfile.file, cfile.line );
+				cfile.err = 1;
+			}
+		}
 		else if (!getopt_helper( &cfile, &gcops, &global_conf ))
 		{
 			error( "%s:%d: unknown section keyword '%s'\n",
diff --git a/src/driver.h b/src/driver.h
index e79a096..3aa30a9 100644
--- a/src/driver.h
+++ b/src/driver.h
@@ -246,6 +246,9 @@ struct driver {
 
 	/* Commit any pending set_msg_flags() commands. */
 	void (*commit_cmds)( store_t *ctx );
+
+	/* Get approximate amount of memory occupied by the driver. */
+	int (*memory_usage)( store_t *ctx );
 };
 
 void free_generic_messages( message_t * );
diff --git a/src/drv_imap.c b/src/drv_imap.c
index 7146d26..2620f49 100644
--- a/src/drv_imap.c
+++ b/src/drv_imap.c
@@ -111,6 +111,7 @@ typedef struct imap_store {
 	int nexttag, num_in_progress;
 	struct imap_cmd *pending, **pending_append;
 	struct imap_cmd *in_progress, **in_progress_append;
+	int buffer_mem; /* memory currently occupied by buffers in the queue */
 
 	/* Used during sequential operations like connect */
 	enum { GreetingPending = 0, GreetingBad, GreetingOk, GreetingPreauth } greeting;
@@ -256,7 +257,10 @@ static void
 done_imap_cmd( imap_store_t *ctx, struct imap_cmd *cmd, int response )
 {
 	cmd->param.done( ctx, cmd, response );
-	free( cmd->param.data );
+	if (cmd->param.data) {
+		free( cmd->param.data );
+		ctx->buffer_mem -= cmd->param.data_len;
+	}
 	free( cmd->cmd );
 	free( cmd );
 }
@@ -299,6 +303,7 @@ send_imap_cmd( imap_store_t *ctx, struct imap_cmd *cmd )
 		iov[1].len = cmd->param.data_len;
 		iov[1].takeOwn = GiveOwn;
 		cmd->param.data = 0;
+		ctx->buffer_mem -= cmd->param.data_len;
 		iov[2].buf = "\r\n";
 		iov[2].len = 2;
 		iov[2].takeOwn = KeepOwn;
@@ -1317,6 +1322,7 @@ imap_socket_read( void *aux )
 				iov[0].len = cmdp->param.data_len;
 				iov[0].takeOwn = GiveOwn;
 				cmdp->param.data = 0;
+				ctx->buffer_mem -= cmdp->param.data_len;
 				iov[1].buf = "\r\n";
 				iov[1].len = 2;
 				iov[1].takeOwn = KeepOwn;
@@ -2507,6 +2513,7 @@ imap_store_msg( store_t *gctx, msg_data_t *data, int to_trash,
 	flagstr[d] = 0;
 
 	INIT_IMAP_CMD(imap_cmd_out_uid, cmd, cb, aux)
+	ctx->buffer_mem += data->len;
 	cmd->gen.param.data_len = data->len;
 	cmd->gen.param.data = data->data;
 	cmd->out_uid = -2;
@@ -2621,6 +2628,16 @@ imap_commit_cmds( store_t *gctx )
 	(void)gctx;
 }
 
+/******************* imap_memory_usage *******************/
+
+static int
+imap_memory_usage( store_t *gctx )
+{
+	imap_store_t *ctx = (imap_store_t *)gctx;
+
+	return ctx->buffer_mem + ctx->conn.buffer_mem;
+}
+
 /******************* imap_parse_store *******************/
 
 imap_server_conf_t *servers, **serverapp = &servers;
@@ -2899,4 +2916,5 @@ struct driver imap_driver = {
 	imap_close_box,
 	imap_cancel_cmds,
 	imap_commit_cmds,
+	imap_memory_usage,
 };
diff --git a/src/drv_maildir.c b/src/drv_maildir.c
index 27073eb..29d4c2e 100644
--- a/src/drv_maildir.c
+++ b/src/drv_maildir.c
@@ -1605,6 +1605,12 @@ maildir_commit_cmds( store_t *gctx )
 	(void) gctx;
 }
 
+static int
+maildir_memory_usage( store_t *gctx ATTR_UNUSED )
+{
+	return 0;
+}
+
 static int
 maildir_parse_store( conffile_t *cfg, store_conf_t **storep )
 {
@@ -1672,4 +1678,5 @@ struct driver maildir_driver = {
 	maildir_close_box,
 	maildir_cancel_cmds,
 	maildir_commit_cmds,
+	maildir_memory_usage,
 };
diff --git a/src/main.c b/src/main.c
index beb6059..f587e2d 100644
--- a/src/main.c
+++ b/src/main.c
@@ -43,6 +43,8 @@ int Pid;		/* for maildir and imap */
 char Hostname[256];	/* for maildir */
 const char *Home;	/* for config */
 
+int BufferLimit = 10 * 1024 * 1024;
+
 static void
 version( void )
 {
diff --git a/src/mbsync.1 b/src/mbsync.1
index e5ad1fb..02e89ad 100644
--- a/src/mbsync.1
+++ b/src/mbsync.1
@@ -583,6 +583,14 @@ This option is meaningless for \fBSyncState\fR if the latter is \fB*\fR,
 obviously. However, it also determines the default of \fBInfoDelimiter\fR.
 (Global default: \fI;\fR on Windows, \fI:\fR everywhere else)
 ..
+.TP
+\fBBufferLimit\fR \fIsize\fR[\fBk\fR|\fBm\fR][\fBb\fR]
+The per-Channel, per-direction instantaneous memory usage above which
+\fBmbsync\fR will refrain from using more memory. Note that this is no
+absolute limit, as even a single message can consume more memory than
+this.
+(Default: \fI10M\fR)
+..
 .SH RECOMMENDATIONS
 Make sure your IMAP server does not auto-expunge deleted messages - it is
 slow, and semantically somewhat questionable. Specifically, Gmail needs to
diff --git a/src/socket.c b/src/socket.c
index b7de54b..5cde674 100644
--- a/src/socket.c
+++ b/src/socket.c
@@ -737,6 +737,7 @@ dispose_chunk( conn_t *conn )
 	buff_chunk_t *bc = conn->write_buf;
 	if (!(conn->write_buf = bc->next))
 		conn->write_buf_append = &conn->write_buf;
+	conn->buffer_mem -= bc->len;
 	free( bc );
 }
 
@@ -770,6 +771,7 @@ static void
 do_append( conn_t *conn, buff_chunk_t *bc )
 {
 	bc->next = 0;
+	conn->buffer_mem += bc->len;
 	*conn->write_buf_append = bc;
 	conn->write_buf_append = &bc->next;
 }
diff --git a/src/socket.h b/src/socket.h
index a420e49..7ea6086 100644
--- a/src/socket.h
+++ b/src/socket.h
@@ -104,6 +104,7 @@ typedef struct {
 	int append_avail; /* space left in accumulating buffer */
 #endif
 	int write_offset; /* offset into buffer head */
+	int buffer_mem; /* memory currently occupied by buffers in the queue */
 
 	/* reading */
 	int offset; /* start of filled bytes in buffer */
diff --git a/src/sync.c b/src/sync.c
index 736f9ca..24d520d 100644
--- a/src/sync.c
+++ b/src/sync.c
@@ -154,6 +154,7 @@ typedef struct {
 	store_t *ctx[2];
 	driver_t *drv[2];
 	const char *orig_name[2];
+	message_t *new_msgs[2];
 	int state[2], ref_count, nsrecs, ret, lfd, existing, replayed;
 	int new_total[2], new_done[2];
 	int flags_total[2], flags_done[2];
@@ -207,6 +208,7 @@ static int check_cancel( sync_vars_t *svars );
 #define ST_CLOSING         (1<<12)
 #define ST_CONFIRMED       (1<<13)
 #define ST_PRESENT         (1<<14)
+#define ST_SENDING_NEW     (1<<15)
 
 
 static void
@@ -1336,7 +1338,6 @@ box_loaded( int sts, void *aux )
 	sync_rec_t *srec;
 	sync_rec_map_t *srecmap;
 	message_t *tmsg;
-	copy_vars_t *cv;
 	flag_vars_t *fv;
 	int uid, no[2], del[2], alive, todel, t1, t2;
 	int sflags, nflags, aflags, dflags, nex;
@@ -1724,21 +1725,7 @@ box_loaded( int sts, void *aux )
 	for (t = 0; t < 2; t++) {
 		svars->newuid[t] = svars->ctx[t]->uidnext;
 		Fprintf( svars->jfp, "%c %d\n", "{}"[t], svars->newuid[t] );
-		for (tmsg = svars->ctx[1-t]->msgs; tmsg; tmsg = tmsg->next) {
-			if ((srec = tmsg->srec) && srec->tuid[0]) {
-				svars->new_total[t]++;
-				stats( svars );
-				cv = nfmalloc( sizeof(*cv) );
-				cv->cb = msg_copied;
-				cv->aux = AUX;
-				cv->srec = srec;
-				cv->msg = tmsg;
-				copy_msg( cv );
-				if (check_cancel( svars ))
-					goto out;
-			}
-		}
-		svars->state[t] |= ST_SENT_NEW;
+		svars->new_msgs[t] = svars->ctx[1-t]->msgs;
 		msgs_copied( svars, t );
 		if (check_cancel( svars ))
 			goto out;
@@ -1809,11 +1796,42 @@ static void sync_close( sync_vars_t *svars, int t );
 static void
 msgs_copied( sync_vars_t *svars, int t )
 {
-	if (!(svars->state[t] & ST_SENT_NEW) || svars->new_done[t] < svars->new_total[t])
+	message_t *tmsg;
+	sync_rec_t *srec;
+	copy_vars_t *cv;
+
+	if (svars->state[t] & ST_SENDING_NEW)
 		return;
 
 	sync_ref( svars );
 
+	if (!(svars->state[t] & ST_SENT_NEW)) {
+		for (tmsg = svars->new_msgs[t]; tmsg; tmsg = tmsg->next) {
+			if ((srec = tmsg->srec) && srec->tuid[0]) {
+				if (svars->drv[t]->memory_usage( svars->ctx[t] ) >= BufferLimit) {
+					svars->new_msgs[t] = tmsg;
+					goto out;
+				}
+				svars->new_total[t]++;
+				stats( svars );
+				svars->state[t] |= ST_SENDING_NEW;
+				cv = nfmalloc( sizeof(*cv) );
+				cv->cb = msg_copied;
+				cv->aux = AUX;
+				cv->srec = srec;
+				cv->msg = tmsg;
+				copy_msg( cv );
+				svars->state[t] &= ~ST_SENDING_NEW;
+				if (check_cancel( svars ))
+					goto out;
+			}
+		}
+		svars->state[t] |= ST_SENT_NEW;
+	}
+
+	if (svars->new_done[t] < svars->new_total[t])
+		goto out;
+
 	Fprintf( svars->jfp, "%c %d\n", ")("[t], svars->maxuid[1-t] );
 	sync_close( svars, 1-t );
 	if (check_cancel( svars ))