Sun Java System Messaging Server 6 2005Q4 MTA Developer's Reference

MTA_ENC_COMPRESSED_BINARY

Compress the data with Gnu zip. No other encoding of the data will be done. This setting may be changed with any of the other MTA_ENC_ item codes. The default encoding is MTA_ENC_UNKNOWN. The item_address and item_length fields are ignored for this item code.