тогда может быть так
Код:
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@echo off
echo %COMPUTERNAME% >name.txt
for /f "delims=-" %%H in ('more +0 "name.txt"') do set name=%%H
del name.txt
echo %name%
pause
весь код
Код:

@echo off
echo %COMPUTERNAME% >name.txt
for /f "delims=-" %%H in ('more +0 "name.txt"') do set name=%%H
del name.txt
set postFrom=mail@mail.ru
set postTo=mail@mail.ru
set postLogin=mail
set postPass=123456789
set postSUB="%name% ERROR"
set post="error"
set postServer=server:25
"%PATHPOSTGRESBACKUP%\exe\mail\sendEmail.exe" -f "%postFrom%" -t "%postTo%" -s "%postServer%" -xu "%postLogin%" -xp "%postPass%" -u "%postSUB%" -m "%post%"