Double negatives in code always confuse me a bit... You have these two conditions:

if (!($MailFrom =~/\@*\./)) {
    $kill = 1;
    print "Invalid return address in From box.
";
}

if (!($MailTo !~/\@*\./)) {
    $kill = 1;
    print "Invalid destination address...
";
}
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The following pseudo-code properly "translates" this logic, so the question is: does it also express your true intent?

• if the "from" parameter string contains at least a single period (dot) character, which optionally follows "@", then it's okay, otherwise, complain about it.
• if the "to" parameter string does not contain a period, which optionally follows "@", then it's okay, otherwise complain about it.

Now if the criterion is supposed to be "@" followed by period, you should not have the asterisk after "@" -- this means "zero or more occurrences" of "@" preceding a period, so both "." and "@." match your /\@*\./ regex (but neither of them looks good to me as an email address).