user trouble

[cemaaiem at 30gigs.com]

I have a mail server with sendmail, last night i send a mail to an user and aparently was received(don`t back rebooted) baut today since this morning, all mails sended to tah user are rebooted and the reason in the reboot mail is "host unknown" but the other users don not have any problem with their mail accounts.
i try to delete and re-create the user, but the same situation.
what could it be?



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