Not receiving forwarded emails

I'm using Outlook.com to try and aggregate emails from other accounts. I have set a particular account to forward emails to my outlook email address. I have limited success with this. I have noticed that I am not receiving all of my forwarded emails. I have spoken to the hosting company of the email and they have looked into the issue and ensure me that they emails have been forwarded and are awaiting delivery.

I seem to mainly be getting the problem when I send emails to the forwarded account from .me and .icloud accounts but I have noticed others not coming through.

How can I get this issue resolved?