The MTA measures message sizes in units of blocks. Units of blocks are used, for instance, when logging message sizes, and for the MTA_FRAGMENT_BLOCKS item code in the mtaEnqueueStart() routine. By default, a block is 1024 bytes. However, sites can change this setting with the BLOCK_SIZE option in the option.dat file.
Programs using the SDK can translate units of bytes to blocks by dividing the number of bytes by the value returned by mtaBlockSize(), that is:
bytes_per_block = mtaBlockSize(); block_limit = byte_limit / bytes_per_block;