Some email addresses may be valid but because they look suspicious or unfamiliar, your server may block the email by sending it straight to the trash, into your SPAM folder, or not allowing it through at all. In many cases, your web hosting provider has this extra security set up. If you are experiencing problems with every email account in your company, chances are, your Web Hosting Provider will need to resolve the issue by unblocking the website that is trying to send you emails. This will correct the issue for everyone in the company.
You also have email account settings which can be modified to block or allow any email addresses you want individually. Decide which email addresses are important to you and add them to your "White List." A "White List" is a list of email addresses that you consider important and safe. A "Black List" is a list of email addresses that you want to block.
The graphic below depicts how to add emails to a white list using Network Solutions. To make the changes log into webmail and go into your "Configuration" settings. Look for SPAM or JUNK mail settings and make the changes there. This process is different for every web host but should not be much different.