WHY

个人认为，楼主无非是想判断磁盘剩余空间是否超过警戒值，没有必要转换成MB，补零就行了。
假设警戒值为 1GB

@echo off
set "alarmValue=1073741824"
for /f "tokens=3" %%i in ('dir /a /-c C:') do set "freeSpace=%%i"
set "s1=00000000000000000000%freeSpace%"
set "s2=00000000000000000000%alarmValue%"
if "%s1:~-20%" LSS "%s2:~-20%" (
    echo;低于警戒值，发邮件
) else (
    echo;空间充足，剩余%freeSpace%字节，警戒值%alarmValue%字节
)
pause
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WHY

一定要转换为MB，这样应该可以：

@echo off & setlocal enabledelayedexpansion
set "alarmValue=1024"
for /f "tokens=3" %%i in ('dir /a /-c C:') do set "freeSpace=%%i"
call :GetMB
if !rt! LSS !alarmValue! (
    echo;低于警戒值，发邮件
) else (
    echo;空间充足，剩余!rt!MB，警戒值!alarmValue!MB
)
pause & exit /b

:GetMB
set "str=!freeSpace!FEDCBA9876543210" & set /a Len=0x!str:~16,1!
for /L %%i in (0 1 !Len!) do (
        set "sang=!yu!!freeSpace:~%%i,1!"
        set /a yu=sang%%1048576,sang/=1048576
        if "!yu!" == "0" set "yu="
        set "rt=!rt!!sang!"
)
for /f "tokens=* delims=0" %%i in ("!rt!") do set /a rt=%%i+0
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