How do I use FTP through email?

When FTP is used in a UNIX environment, there is an often-ignored but valuable command, "reget" (meaning "get again") that will cause an interrupted "get" command to be continued, hopefully to completion, after a communications interruption. The principle is obvious - the receiving station has a record of what it got, so it can spool through the file at the sending station and re-start at the right place for a seamless splice. The converse would be "reput" but is not available. Again, the principle is obvious: The sending station does not know how much of the file was actually received, so it would not know where to start.