All mail from a contact suddenly return to them, undelivered and marked "blocked for abuse"

About 3 weeks ago, all e-mails from a long-standing contact (...co.uk) are being returned to

that person and are marked "blocked for abuse." That person can send me messages

using an alternative address, and that person receives all my messages. The contact's

address in not in my blocked contacts' list. It is emply. And there are no top-level (i.e. country

level) receipenats blocked. I added the contact's address

to my preferred contacts' list. Nothing seems to help. Can my ISP, AT&ampT, block an

address at the server level? If so, how do I address that issue. Thanks!!

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