Does your host allow you to forward to two different email addresses? If so, create a Hotmail or Yahoo account and also forward the email there. If it goes missing from both your ISP and the added email account, then the problem is with your host. If it only fails towards your Outlook client, the problem is with your ISP.

If your host does not allow more than one forwarding address per email alias, then just switch the forwarding to a Hotmail or Yahoo account and see, if the trouble still continues. If it does, the trouble is probably your host. If it doesn't, then it is your ISP.

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