well that's the thing, I'm using strict & warnings, and I've tested it extensively using localhost, dumping the messages to the browser instead of using $msg->send. There's absolutely no difference in the output if I play around with the addresses in this environment.

Now that it's on the real server, I pass the email address to send to in the form, so by changing the address I can see it working (for my email address) and not working (for my hotmail address and the address I want to send to).

The script thinks everything is hunkydory, but then, my only check is this: $msg->send   || do { $error ='unsent'; return; };

it looks like I shall have to pester Frankfurt Dirk for the sendmail configuration, because (hubris/a yet worse sin) I'm damned if it's my script. This time.

While it is not the problem, (but as an aside), you would be well off to consider writing that with an 'or' rather than a '||':

$msg->send() or {...};
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The '||' operator binds pretty tightly as it says in the good ol' Camel. (Though not as tightly as the 'arrow' -> operator, if I recall correctly.)