Muchas gracias: recordabas bien
En la documentación de los componentes Indy, encontramos:
TIdPOP3.Delete Method
Código Delphi
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function Delete( const MsgNum: Integer): Boolean;
Delete is a Boolean function that marks a message on the POP3 server for deletion using the message number specified in MsgNum. If a non-existent message number is used, an exception is raised.
Delete uses SendCmd to execute the POP DELE command that marks the indicated message number for deletion. Messages marked for deletion on the POP3 server will be physically removed when Disconnect is called. Use Reset to clear all message deletion marks anytime prior to calling Disconnect.
Delete returns True when the result for the operation, as indicated by LastCmdResult, contains the value ST_OK.