When you're using Net::SMTP, you may get gobs of information by turning the debug on. This way you're going to see all conversation between your Perl script and the server and may eventually figure out what's going wrong.

I think the only change you need to do is:

MIME::Lite->send('smtp', $mail_host, Timeout=>60, Debug=>1);
                                                ^^^^^^^^^^
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even though I've never used MIME::Lite, but I think it should only pass the arguments Timeout=>60, Debug=>1 to Net::SMTP constructor and then start being verbose.