Not receiving emails

My hotmail account, now outlook, is not receiving all the emails that I get sent. I have noticed this when people have sent group emails and everybody has received it excluding me. I have also been told by certain people that when trying to mail me they have to try to send it numerous times before it works (my email being the only problem they have when sending. Occasionaly over the last few months, i have received an email from my own account in the middle of the night with a document attached. These emails I have received through my blackberry. I have not opened the documents.
Do you know how i can fix this? My work life is based on this account and so I am very desperate! Please help!
many thanks