Pop is a procedure used to remove the boundary marker and parent messge part identifier at the top of the internal marker list. 

Calling Pop when no boundary marker values have been Pushed onto the list can result in an "index out of range" exception. 

Use Clear to remove all boundary markers and parent message part identifiers in the internal list. 

Use Push to add a new boundary marker and parent message part identifier to the internal list.

TIdMIMEBoundary.Push 

TIdMIMEBoundary.Clear