You can declare it in the body using the "From:" SMTP header, but also look at the BLAT "-i" and "-f" options which allow you to specify the sender, or use the more granular envelope options that are available.

From the BLAT documentation:
 Originally Posted By: BLAT documentation
Note that if the '-i' option is used, <sender> is included in 'Reply-to:'
and 'Sender:' fields in the header of the message.

Optionally, the following options can be used instead of the -f and -i
options:

-mailfrom <addr> The RFC 821 MAIL From: statement
-from <addr> The RFC 822 From: statement
-replyto <addr> The RFC 822 Reply-To: statement
-returnpath <addr> The RFC 822 Return-Path: statement
-sender <addr> The RFC 822 Sender: statement

For backward consistency, the -f and -i options have precedence over these
RFC 822 defined options. If both -f and -i options are omitted then the
RFC 821 MAIL FROM statement will be defaulted to use the installation-defined
default sender address.


For example you could:
 Code:
blat -i %USERNAME%@%COMPUTERNAME% ...