It specifically means that the instruction is contained with a program block, task block, or stage that has run its termination logic while the instruction was still active. Program blocks run termination on the scan following the EXIT or HALT that stopped it. Task blocks run termination on the scan following completion if run once, or on the scan following the ENTASK enable dropping if run continuously. Stages run termination on the scan following a jump to another stage.
There are probably some other special cases I can't remember off the top of my head, but the key point is that the EMAIL is in a code element that has been allowed to terminate before it was completed. Once you start an EMAIL, don't allow the containing code to terminate until either the Complete bit or Error bit turns on (or the stage transitions for complete and error).