diff --git a/src/utility/stream.cpp b/src/utility/stream.cpp
index f2a89a55..48869958 100644
--- a/src/utility/stream.cpp
+++ b/src/utility/stream.cpp
@@ -32,6 +32,14 @@ namespace vmime {
 namespace utility {
 
 
+// stream
+
+const stream::size_type stream::getBlockSize() const
+{
+	return 32768;  // 32 KB
+}
+
+
 // Helpers
 
 outputStream& operator<<(outputStream& os, const stream::value_type c)
@@ -57,7 +65,12 @@ const stream::size_type bufferedStreamCopy(inputStream& is, outputStream& os)
 const stream::size_type bufferedStreamCopy(inputStream& is, outputStream& os,
 	const stream::size_type length, progressListener* progress)
 {
-	stream::value_type buffer[16384];
+	const stream::size_type blockSize =
+		std::min(is.getBlockSize(), os.getBlockSize());
+
+	std::vector <stream::value_type> vbuffer(blockSize);
+
+	stream::value_type* buffer = &vbuffer.front();
 	stream::size_type total = 0;
 
 	if (progress != NULL)
@@ -443,6 +456,13 @@ void outputStreamSocketAdapter::flush()
 }
 
 
+const stream::size_type outputStreamSocketAdapter::getBlockSize() const
+{
+	return 16384;  // 16 KB
+}
+
+
+
 // inputStreamSocketAdapter
 
 inputStreamSocketAdapter::inputStreamSocketAdapter(net::socket& sok)
@@ -479,6 +499,12 @@ const stream::size_type inputStreamSocketAdapter::skip
 }
 
 
+const stream::size_type inputStreamSocketAdapter::getBlockSize() const
+{
+	return 16384;  // 16 KB
+}
+
+
 #endif // VMIME_HAVE_MESSAGING_FEATURES
 
 
diff --git a/vmime/utility/stream.hpp b/vmime/utility/stream.hpp
index 87c1c7fe..b8d0d189 100644
--- a/vmime/utility/stream.hpp
+++ b/vmime/utility/stream.hpp
@@ -71,6 +71,13 @@ public:
 	/** Type used for lengths in streams.
 	  */
 	typedef string::size_type size_type;
+
+	/** Return the preferred maximum block size when reading
+	  * from or writing to this stream.
+	  *
+	  * @return block size, in bytes
+	  */
+	virtual const size_type getBlockSize() const;
 };
 
 
@@ -395,6 +402,8 @@ public:
 	void write(const value_type* const data, const size_type count);
 	void flush();
 
+	const size_type getBlockSize() const;
+
 private:
 
 	outputStreamSocketAdapter(const outputStreamSocketAdapter&);
@@ -417,6 +426,8 @@ public:
 	const size_type read(value_type* const data, const size_type count);
 	const size_type skip(const size_type count);
 
+	const size_type getBlockSize() const;
+
 private:
 
 	inputStreamSocketAdapter(const inputStreamSocketAdapter&);