In an effort to protect their users, most email providers like Hotmail, Gmail, BTInternet etc automatically direct emails they suspect might be spam directly into your email spam (or junk) folders.
It is always good practice to regularly check and clear your spam folder but if you find any emails that should NOT be flagged as spam, you can quickly ensure that all future emails from that sender arrive safely in your inbox by clicking the 'Not Spam' or 'Not Junk' button. Sometimes this is appears as a 'thumbs up' or 'thumbs down' icon.
To be extra sure that you receive the emails you want, you can 'whitelist' email address by adding them to your contacts and address books.
This microsoft forum has some answers:
Option A - Add the Email Address to the safe senders list
1. On your inbox, click the gear icon and select More mail settings on the dropdown list.
2. Under Preventing junk email, click Safe and blocked senders.
3. Click Safe Senders.
4. In the text box, type in the email address or domain you’d like to always receive email from, and then click Add to list.
Option B - Check your Email Rules
1. On your inbox, click the gear icon and select More mail settings on the dropdown list.
2. Under Customizing Outlook, click Rules for sorting new messages.
3. Perform one of the actions as per your requirement:
Click Edit next to the rule that you want to edit.
Or
Choose the rule that you want to delete and click Delete.
Note: Not all international characters are supported as filters.
Option C - Check your Junk Mail settings
1. On your inbox, click the gear icon and select More mail settings on the dropdown list.
2. Under Preventing junk email, click Filters and reporting.
3. Under Choose a junk e-mail filter, choose the level of protection that you want, and then click Save. If the Exclusive level was set, messages from senders not on your Contacts or Safe senders lists will have been sent to the Junk e-mail folder.
Because there are so many email providers and computer combinations, the best way to find out how to do this is to Google the question 'How to add email sender to my address book [EMAIL PROVIDER] [COMPUTER SYSTEM]' Where you enter your email provider (e.g. Yahoo) and computer system (e.g. Windows 7 PC).
Once you have done this, all future emails from those providers should never be misdirected again.