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Blat cannot do this, it does not make any attempt to parse
your message body. What you can do is define your body text
as command line text instead of using a file, then your
batch file will be able to automatically change the %
variables based on environment variable values.

Blat - -body "Actually, you have %number% files bigger then
on your %drive%." -s "Subject line" -t "emailaddress"