I guess you are using Outlook (express?) If so, delete everything in your inbox. That should solve it. If not you can remove inbox.dbx. This is a database file where your incomming mails are stored. You may need to repair Outlook after doing so.

Tip: If you need help, provide as much relevant details as possible. We can’t give accurate help if we have to guess/assume many things. Think of things like:
What os are you using?
What version of Avast?
What vps version?
Where/when does a error occure?
What is the exact error message?
What have you tried/done sofar to solve it?
etc etc.