Not all email received in Outlook

I am new to DH and have a client problem. They are receiving some but not all their email in Outlook. They have tried in both POP and IMAP settings. They are able to see all the mail in webmail. I have had reseller accounts with other hosts before, but I have never encountered this one.

Usually when only some emails come through, there is some kind of spam blocker that removes the other emails. This could be either server based or outlook based. Not sure about Outlook, but outlook express also has a delete email if the address matches the black list.
Hope this gives you some ideas,

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