I understood that, occasionally, an ISP will be blacklisted if there is an unusually high level of spam coming from their customers. Tiscali springs to mind as an offender. After a few weeks, if they've cleaned up, then mail from their domain will be accepted again.
But I'm inclined to believe the simple explanation, as already put forward, finger trouble at the receiving end. The friend's speculation is being taking as fact rather than speculation unfortunately.
JH
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