lib-smtp: client: Increase the default BDAT chunk size.

Reduces the number of roundtrips and keeps the TCP window filled better.
The default is now based on the NET_BLOCK_SIZE definition.

src/lib-smtp/smtp-client.h

@@ -12,7 +12,7 @@ struct smtp_client_request;
 #define SMTP_DEFAULT_CONNECT_TIMEOUT_MSECS (1000*30)
 #define SMTP_DEFAULT_COMMAND_TIMEOUT_MSECS (1000*60*5)
 #define SMTP_DEFAULT_MAX_REPLY_SIZE ((size_t)-1)
-#define SMTP_DEFAULT_MAX_DATA_CHUNK_SIZE IO_BLOCK_SIZE
+#define SMTP_DEFAULT_MAX_DATA_CHUNK_SIZE NET_BLOCK_SIZE
 #define SMTP_DEFAULT_MAX_DATA_CHUNK_PIPELINE 4
 
 enum smtp_client_command_error {