That is because you never set the sender address. I believe that adding the following line to your code may do it (I'd need to dust off my "pipe to sendmail" hat to say for certain).

print MAIL "Sender: addrtogetbounces\@example.com\n";
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You could solve this problem and enable for better error checking if you used SNMP to send the message, instead of forking a pipe to a sendmail process. There are sevearl good modules to use to do this : Net::SMTP, Mail::Sendmail, even MIME::Lite has this capability...