As attachments get larger, and end users use email rather than ftp for file transfer for convenience sake, a UDP implementation, perhaps using UDP as a data streaming channel could become a very useful configuration, and the transfer speed over high latency links (think satellite etc) could improve immensely.
Not necessarily. SMTP is not DNS or SNMP. A SMTP communication requires far more than two ip packets (question/answer). So, as some others already mentioned, for reliable submission the software has to ensure correct integrity and order of the submitted ip packets - even over unreliable and asynchronous media. To be in advantage over smtp/tcp those checks have to be faster than the highly optimised ones in your tcp stack.
Jan
