Answer
CHECKPOINT is the ESMTP keyword controlling whether or not a server supports SMTP Restart. That is, the ability to continue an interrupted message transmission from the point it was interrupted without requiring the complete message to be re-transmitted.
The benefits of CHECKPOINT become more apparent if either one of the sender or the recipient is on a poor or unreliable connection. Take as an example a client on a low quality dialup connection transmitting a 10 MB message which fails 9.5 MB into transmission. The next time the client connects to transmit it will only have a further 0.5 MB to transmit, not the complete 10 MB.
Bandwidth is also often an issue with these types of connections, and if large messages are being trasnmitted then not having to restart any aborted transmissions from the beginning can produce a considerable saving on bandwidth.
See Also:
Keywords:checkpoint smtp restart