diff --git a/NEWS b/NEWS
index d84eb62..fe8c6f5 100644
--- a/NEWS
+++ b/NEWS
@@ -10,6 +10,8 @@ Support for SASL (flexible authentication) has been added.
 
 Support for Windows file systems has been added.
 
+Support for compressed data transfer has been added.
+
 [1.1.0]
 
 Support for hierarchical mailboxes in Patterns.
diff --git a/configure.ac b/configure.ac
index ac696ee..7b4086d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -150,6 +150,15 @@ if test "x$ac_cv_berkdb4" = xno; then
   AC_MSG_ERROR([Berkley DB >= 4.2 not found.])
 fi
 
+have_zlib=
+AC_CHECK_LIB([z], [deflate],
+    [AC_CHECK_HEADER(zlib.h,
+        [have_zlib=1
+         AC_SUBST([Z_LIBS], ["-lz"])
+         AC_DEFINE([HAVE_LIBZ], 1, [if you have the zlib library])]
+     )]
+)
+
 AC_ARG_ENABLE(compat,
   AC_HELP_STRING([--disable-compat], [don't include isync compatibility wrapper [no]]),
   [ob_cv_enable_compat=$enableval])
@@ -172,4 +181,9 @@ if test -n "$have_sasl_paths"; then
 else
     AC_MSG_RESULT([Not using SASL])
 fi
+if test -n "$have_zlib"; then
+    AC_MSG_RESULT([Using zlib])
+else
+    AC_MSG_RESULT([Not using zlib])
+fi
 AC_MSG_RESULT()
diff --git a/src/Makefile.am b/src/Makefile.am
index 8ae3b53..0a49f21 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -6,7 +6,7 @@ SUBDIRS = $(compat_dir)
 bin_PROGRAMS = mbsync mdconvert
 
 mbsync_SOURCES = main.c sync.c config.c util.c socket.c driver.c drv_imap.c drv_maildir.c
-mbsync_LDADD = -ldb $(SSL_LIBS) $(SOCK_LIBS) $(SASL_LIBS)
+mbsync_LDADD = -ldb $(SSL_LIBS) $(SOCK_LIBS) $(SASL_LIBS) $(Z_LIBS)
 noinst_HEADERS = common.h config.h driver.h sync.h socket.h
 
 mdconvert_SOURCES = mdconvert.c
diff --git a/src/drv_imap.c b/src/drv_imap.c
index 141dc40..a931ce9 100644
--- a/src/drv_imap.c
+++ b/src/drv_imap.c
@@ -196,7 +196,8 @@ enum CAPABILITY {
 	UIDPLUS,
 	LITERALPLUS,
 	MOVE,
-	NAMESPACE
+	NAMESPACE,
+	COMPRESS_DEFLATE
 };
 
 static const char *cap_list[] = {
@@ -210,7 +211,8 @@ static const char *cap_list[] = {
 	"UIDPLUS",
 	"LITERAL+",
 	"MOVE",
-	"NAMESPACE"
+	"NAMESPACE",
+	"COMPRESS=DEFLATE"
 };
 
 #define RESP_OK       0
@@ -1486,6 +1488,9 @@ static void imap_open_store_authenticate2_p2( imap_store_t *, struct imap_cmd *,
 static void imap_open_store_namespace( imap_store_t * );
 static void imap_open_store_namespace_p2( imap_store_t *, struct imap_cmd *, int );
 static void imap_open_store_namespace2( imap_store_t * );
+#ifdef HAVE_LIBZ
+static void imap_open_store_compress_p2( imap_store_t *, struct imap_cmd *, int );
+#endif
 static void imap_open_store_finalize( imap_store_t * );
 #ifdef HAVE_LIBSSL
 static void imap_open_store_ssl_bail( imap_store_t * );
@@ -2041,12 +2046,32 @@ imap_open_store_namespace2( imap_store_t *ctx )
 			ctx->prefix = nsp_1st_ns->val;
 		if (!ctx->delimiter)
 			ctx->delimiter = nfstrdup( nsp_1st_dl->val );
+#ifdef HAVE_LIBZ
+		if (CAP(COMPRESS_DEFLATE)) { /* XXX make that configurable */
+			imap_exec( ctx, 0, imap_open_store_compress_p2, "COMPRESS DEFLATE" );
+			return;
+		}
+#endif
 		imap_open_store_finalize( ctx );
 	} else {
 		imap_open_store_bail( ctx );
 	}
 }
 
+#ifdef HAVE_LIBZ
+static void
+imap_open_store_compress_p2( imap_store_t *ctx, struct imap_cmd *cmd ATTR_UNUSED, int response )
+{
+	if (response == RESP_NO) {
+		/* We already reported an error, but it's not fatal to us. */
+		imap_open_store_finalize( ctx );
+	} else if (response == RESP_OK) {
+		socket_start_deflate( &ctx->conn );
+		imap_open_store_finalize( ctx );
+	}
+}
+#endif
+
 static void
 imap_open_store_finalize( imap_store_t *ctx )
 {
diff --git a/src/socket.c b/src/socket.c
index 586c98c..b300910 100644
--- a/src/socket.c
+++ b/src/socket.c
@@ -280,6 +280,43 @@ static void start_tls_p3( conn_t *conn, int ok )
 
 #endif /* HAVE_LIBSSL */
 
+#ifdef HAVE_LIBZ
+
+static void z_fake_cb( void * );
+
+void
+socket_start_deflate( conn_t *conn )
+{
+	int result;
+
+	conn->in_z = nfcalloc( sizeof(*conn->in_z) );
+	result = inflateInit2(
+			conn->in_z,
+			-15 /* Use raw deflate */
+		);
+	if (result != Z_OK) {
+		error( "Fatal: Cannot initialize decompression: %s\n", conn->in_z->msg );
+		abort();
+	}
+
+	conn->out_z = nfcalloc( sizeof(*conn->out_z) );
+	result = deflateInit2(
+			conn->out_z,
+			Z_DEFAULT_COMPRESSION, /* Compression level */
+			Z_DEFLATED, /* Only valid value */
+			-15, /* Use raw deflate */
+			8, /* Default memory usage */
+			Z_DEFAULT_STRATEGY /* Don't try to do anything fancy */
+		);
+	if (result != Z_OK) {
+		error( "Fatal: Cannot initialize compression: %s\n", conn->out_z->msg );
+		abort();
+	}
+
+	init_wakeup( &conn->z_fake, z_fake_cb, conn );
+}
+#endif /* HAVE_LIBZ */
+
 static void socket_fd_cb( int, void * );
 static void socket_fake_cb( void * );
 
@@ -500,6 +537,17 @@ socket_close( conn_t *sock )
 		sock->ssl = 0;
 		wipe_wakeup( &sock->ssl_fake );
 	}
+#endif
+#ifdef HAVE_LIBZ
+	if (sock->in_z) {
+		inflateEnd( sock->in_z );
+		free( sock->in_z );
+		sock->in_z = 0;
+		deflateEnd( sock->out_z );
+		free( sock->out_z );
+		sock->out_z = 0;
+		wipe_wakeup( &sock->z_fake );
+	}
 #endif
 	while (sock->write_buf)
 		dispose_chunk( sock );
@@ -507,23 +555,30 @@ socket_close( conn_t *sock )
 	sock->append_buf = 0;
 }
 
-static void
-socket_fill( conn_t *sock )
+static int
+prepare_read( conn_t *sock, char **buf, int *len )
 {
-	char *buf;
 	int n = sock->offset + sock->bytes;
-	int len = sizeof(sock->buf) - n;
-	if (!len) {
+	if (!(*len = sizeof(sock->buf) - n)) {
 		error( "Socket error: receive buffer full. Probably protocol error.\n" );
 		socket_fail( sock );
-		return;
+		return -1;
 	}
+	*buf = sock->buf + n;
+	return 0;
+}
+
+static int
+do_read( conn_t *sock, char *buf, int len )
+{
+	int n;
+
 	assert( sock->fd >= 0 );
-	buf = sock->buf + n;
 #ifdef HAVE_LIBSSL
 	if (sock->ssl) {
 		if ((n = ssl_return( "read from", sock, SSL_read( sock->ssl, buf, len ) )) <= 0)
-			return;
+			return n;
+
 		if (n == len && SSL_pending( sock->ssl ))
 			conf_wakeup( &sock->ssl_fake, 0 );
 	} else
@@ -532,15 +587,71 @@ socket_fill( conn_t *sock )
 		if ((n = read( sock->fd, buf, len )) < 0) {
 			sys_error( "Socket error: read from %s", sock->name );
 			socket_fail( sock );
-			return;
 		} else if (!n) {
 			error( "Socket error: read from %s: unexpected EOF\n", sock->name );
 			socket_fail( sock );
-			return;
+			return -1;
 		}
 	}
-	sock->bytes += n;
-	sock->read_callback( sock->callback_aux );
+
+	return n;
+}
+
+#ifdef HAVE_LIBZ
+static void
+socket_fill_z( conn_t *sock )
+{
+	char *buf;
+	int len;
+
+	if (prepare_read( sock, &buf, &len ) < 0)
+		return;
+
+	sock->in_z->avail_out = len;
+	sock->in_z->next_out = (unsigned char *)buf;
+
+	if (inflate( sock->in_z, Z_SYNC_FLUSH ) != Z_OK) {
+		error( "Error decompressing data from %s: %s\n", sock->name, sock->in_z->msg );
+		socket_fail( sock );
+		return;
+	}
+
+	if (!sock->in_z->avail_out)
+		conf_wakeup( &sock->z_fake, 0 );
+
+	if ((len = (char *)sock->in_z->next_out - buf)) {
+		sock->bytes += len;
+		sock->read_callback( sock->callback_aux );
+	}
+}
+#endif
+
+static void
+socket_fill( conn_t *sock )
+{
+#ifdef HAVE_LIBZ
+	if (sock->in_z) {
+		/* The timer will preempt reads until the buffer is empty. */
+		assert( !sock->in_z->avail_in );
+		sock->in_z->next_in = (uchar *)sock->z_buf;
+		if ((sock->in_z->avail_in = do_read( sock, sock->z_buf, sizeof(sock->z_buf) )) <= 0)
+			return;
+		socket_fill_z( sock );
+	} else
+#endif
+	{
+		char *buf;
+		int len;
+
+		if (prepare_read( sock, &buf, &len ) < 0)
+			return;
+
+		if ((len = do_read( sock, buf, len )) <= 0)
+			return;
+
+		sock->bytes += len;
+		sock->read_callback( sock->callback_aux );
+	}
 }
 
 int
@@ -655,6 +766,49 @@ do_append( conn_t *conn, buff_chunk_t *bc )
  * sufficiently small to keep SSL latency low with a slow uplink. */
 #define WRITE_CHUNK_SIZE 1024
 
+static void
+do_flush( conn_t *conn )
+{
+	buff_chunk_t *bc = conn->append_buf;
+#ifdef HAVE_LIBZ
+	if (conn->out_z) {
+		int buf_avail = conn->append_avail;
+		do {
+			if (!bc) {
+				buf_avail = WRITE_CHUNK_SIZE;
+				bc = nfmalloc( offsetof(buff_chunk_t, data) + buf_avail );
+				bc->len = 0;
+			}
+			conn->out_z->next_in = Z_NULL;
+			conn->out_z->avail_in = 0;
+			conn->out_z->next_out = (uchar *)bc->data + bc->len;
+			conn->out_z->avail_out = buf_avail;
+			if (deflate( conn->out_z, Z_PARTIAL_FLUSH ) != Z_OK) {
+				error( "Fatal: Compression error: %s\n", conn->out_z->msg );
+				abort();
+			}
+			bc->len = (char *)conn->out_z->next_out - bc->data;
+			if (bc->len) {
+				do_append( conn, bc );
+				bc = 0;
+				buf_avail = 0;
+			} else {
+				buf_avail = conn->out_z->avail_out;
+			}
+		} while (!conn->out_z->avail_out);
+		conn->append_buf = bc;
+		conn->append_avail = buf_avail;
+	} else
+#endif
+	if (bc) {
+		do_append( conn, bc );
+		conn->append_buf = 0;
+#ifdef HAVE_LIBZ
+		conn->append_avail = 0;
+#endif
+	}
+}
+
 int
 socket_write( conn_t *conn, conn_iovec_t *iov, int iovcnt )
 {
@@ -663,29 +817,54 @@ socket_write( conn_t *conn, conn_iovec_t *iov, int iovcnt )
 
 	for (i = 0; i < iovcnt; i++)
 		total += iov[i].len;
-	bc = conn->append_buf;
-	if (bc && total >= WRITE_CHUNK_SIZE) {
+	if (total >= WRITE_CHUNK_SIZE) {
 		/* If the new data is too big, queue the pending buffer to avoid latency. */
-		do_append( conn, bc );
-		bc  = 0;
+		do_flush( conn );
 	}
+	bc = conn->append_buf;
+#ifdef HAVE_LIBZ
+	buf_avail = conn->append_avail;
+#endif
 	while (total) {
 		if (!bc) {
+			/* We don't do anything special when compressing, as there is no way to
+			 * predict a reasonable output buffer size anyway - deflatePending() does
+			 * not account for consumed but not yet compressed input, and adding up
+			 * the deflateBound()s would be a tad *too* pessimistic. */
 			buf_avail = total > WRITE_CHUNK_SIZE ? total : WRITE_CHUNK_SIZE;
 			bc = nfmalloc( offsetof(buff_chunk_t, data) + buf_avail );
 			bc->len = 0;
+#ifndef HAVE_LIBZ
 		} else {
 			/* A pending buffer will always be of standard size - over-sized
 			 * buffers are immediately filled and queued. */
 			buf_avail = WRITE_CHUNK_SIZE - bc->len;
+#endif
 		}
 		while (total) {
 			len = iov->len - offset;
-			if (len > buf_avail)
-				len = buf_avail;
-			memcpy( bc->data + bc->len, iov->buf + offset, len );
-			bc->len += len;
-			buf_avail -= len;
+#ifdef HAVE_LIBZ
+			if (conn->out_z) {
+				conn->out_z->next_in = (uchar *)iov->buf + offset;
+				conn->out_z->avail_in = len;
+				conn->out_z->next_out = (uchar *)bc->data + bc->len;
+				conn->out_z->avail_out = buf_avail;
+				if (deflate( conn->out_z, Z_NO_FLUSH ) != Z_OK) {
+					error( "Fatal: Compression error: %s\n", conn->out_z->msg );
+					abort();
+				}
+				bc->len = (char *)conn->out_z->next_out - bc->data;
+				buf_avail = conn->out_z->avail_out;
+				len -= conn->out_z->avail_in;
+			} else
+#endif
+			{
+				if (len > buf_avail)
+					len = buf_avail;
+				memcpy( bc->data + bc->len, iov->buf + offset, len );
+				bc->len += len;
+				buf_avail -= len;
+			}
 			offset += len;
 			total -= len;
 			if (offset == iov->len) {
@@ -702,8 +881,16 @@ socket_write( conn_t *conn, conn_iovec_t *iov, int iovcnt )
 		}
 	}
 	conn->append_buf = bc;
+#ifdef HAVE_LIBZ
+	conn->append_avail = buf_avail;
+#endif
 	/* Queue the pending write once the main loop goes idle. */
-	conf_wakeup( &conn->fd_fake, bc ? 0 : -1 );
+	conf_wakeup( &conn->fd_fake,
+#ifdef HAVE_LIBZ
+	             /* Always give zlib a chance to flush its internal buffer. */
+	             conn->out_z ||
+#endif
+	             bc ? 0 : -1 );
 	/* If no writes were queued before, ensure that flushing commences. */
 	if (!exwb)
 		return do_queued_write( conn );
@@ -763,13 +950,22 @@ socket_fake_cb( void *aux )
 	conn_t *conn = (conn_t *)aux;
 
 	buff_chunk_t *exwb = conn->write_buf;
-	do_append( conn, conn->append_buf );
-	conn->append_buf = 0;
+	do_flush( conn );
 	/* If no writes were queued before, ensure that flushing commences. */
 	if (!exwb)
 		do_queued_write( conn );
 }
 
+#ifdef HAVE_LIBZ
+static void
+z_fake_cb( void *aux )
+{
+	conn_t *conn = (conn_t *)aux;
+
+	socket_fill_z( conn );
+}
+#endif
+
 #ifdef HAVE_LIBSSL
 static void
 ssl_fake_cb( void *aux )
diff --git a/src/socket.h b/src/socket.h
index efecce6..a420e49 100644
--- a/src/socket.h
+++ b/src/socket.h
@@ -25,6 +25,10 @@
 
 #include "common.h"
 
+#ifdef HAVE_LIBZ
+#include <zlib.h>
+#endif
+
 #ifdef HAVE_LIBSSL
 typedef struct ssl_st SSL;
 typedef struct ssl_ctx_st SSL_CTX;
@@ -76,6 +80,10 @@ typedef struct {
 	SSL *ssl;
 	wakeup_t ssl_fake;
 #endif
+#ifdef HAVE_LIBZ
+	z_streamp in_z, out_z;
+	wakeup_t z_fake;
+#endif
 
 	void (*bad_callback)( void *aux ); /* async fail while sending or listening */
 	void (*read_callback)( void *aux ); /* data available for reading */
@@ -92,6 +100,9 @@ typedef struct {
 	/* writing */
 	buff_chunk_t *append_buf; /* accumulating buffer */
 	buff_chunk_t *write_buf, **write_buf_append; /* buffer head & tail */
+#ifdef HAVE_LIBZ
+	int append_avail; /* space left in accumulating buffer */
+#endif
 	int write_offset; /* offset into buffer head */
 
 	/* reading */
@@ -99,6 +110,9 @@ typedef struct {
 	int bytes; /* number of filled bytes in buffer */
 	int scanoff; /* offset to continue scanning for newline at, relative to 'offset' */
 	char buf[100000];
+#ifdef HAVE_LIBZ
+	char z_buf[100000];
+#endif
 } conn_t;
 
 /* call this before doing anything with the socket */
@@ -120,6 +134,7 @@ static INLINE void socket_init( conn_t *conn,
 }
 void socket_connect( conn_t *conn, void (*cb)( int ok, void *aux ) );
 void socket_start_tls(conn_t *conn, void (*cb)( int ok, void *aux ) );
+void socket_start_deflate( conn_t *conn );
 void socket_close( conn_t *sock );
 int socket_read( conn_t *sock, char *buf, int len ); /* never waits */
 char *socket_read_line( conn_t *sock ); /* don't free return value; never waits */