diff --git a/MailKit/ByteArrayBuilder.cs b/MailKit/ByteArrayBuilder.cs
index 43e5b8b3ce..c080c17938 100644
--- a/MailKit/ByteArrayBuilder.cs
+++ b/MailKit/ByteArrayBuilder.cs
@@ -27,17 +27,20 @@
 using System;
 using System.Text;
 using System.Buffers;
+using System.Runtime.CompilerServices;
 
 namespace MailKit
 {
 	class ByteArrayBuilder : IDisposable
 	{
+		readonly int initialCapacity;
 		byte[] buffer;
 		int length;
 
-		public ByteArrayBuilder (int initialCapacity)
+		public ByteArrayBuilder (int capacity)
 		{
-			buffer = ArrayPool<byte>.Shared.Rent (initialCapacity);
+			buffer = ArrayPool<byte>.Shared.Rent (capacity);
+			initialCapacity = capacity;
 			length = 0;
 		}
 
@@ -54,6 +57,7 @@ public byte[] GetBuffer ()
 			return buffer;
 		}
 
+		[MethodImpl (MethodImplOptions.AggressiveInlining)]
 		void EnsureCapacity (int capacity)
 		{
 			if (capacity > buffer.Length) {
@@ -79,6 +83,11 @@ public void Append (byte[] text, int startIndex, int count)
 
 		public void Clear ()
 		{
+			if (buffer.Length > initialCapacity * 4) {
+				ArrayPool<byte>.Shared.Return (buffer);
+				buffer = ArrayPool<byte>.Shared.Rent (initialCapacity);
+			}
+
 			length = 0;
 		}