Checkpoint provides a facility by which a messaging server and client can uniquely identify a particular SMTP transaction. The server then stores this identifying information along with all the information it receives as the transaction proceeds.
If the transaction is interrupted during the data transfer phase the SMTP client may establish a new SMTP session at a later time and ask the server to continue the transaction from the point where the server lost its connection with the client.
The server then reestablishes the transaction context and tells the client where to resume operations. If this is acceptable the client resumes operations at this point.
If the server regularly receives large email messages the use of Checkpoint between the server and the sending clients can save considerable amounts of bandwidth. It is particularly useful where the server is on a slow or unreliable connection.
Checkpoint is also sometimes referred to as Restart.