tente substituir prerotate
por firstaction
. Desta forma, o seu comando de correio só será executado uma vez para todos os registros definidos em um bloco.
este é o texto da página man:
firstaction/endscript
The lines between firstaction and endscript (both of which must appear on lines by themselves) are executed once before all log files that match the
wildcarded pattern are rotated, before prerotate script is run and only if at least one log will actually be rotated. These directives may only
appear inside a log file definition. If the script exits with error, no further processing is done. See also lastaction.