MX records ring a bell. This is what your server uses to find the remote user's server. I think they are server based, so it extracts the destination (e.g. virginmedia.com) from your email then looks it up in it's MX records. If no record is found, it doesn't get delivered.
You could try sending to another user on the same domain (@xyz.com) if you know one.
Or the remote user may have inadvertently blacklisted you.
I don't know if turning on delivery confirmation would result in you getting an error message back, which would be helpful.
Best bet might be to research on your ISPs support page, or place a call with them.
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