This applies to Exchange Email Setups running under the Exchange Server 2010 Setup
Before you go in and try to reach your exchange admins to look at this issue.
Kindly first check your contacts folder in outlook. Do you have more than 1000+ contacts? The error may seem to be cropping from this; though its not your fault.
Under Options – Click Junk Email – Under Safe Senders Tab – Remove the Tick Mark against “Also Trust e-mail from my contacts”
With this option enabled, Outlook attempts to add all of the email addresses for your contacts to the Safe Senders list. So if you are having thousands in your contacts folder; your limit on the exchange server for such information which is by default 510kb is exhausted.
If this option was unticked and you still continue to face the problem; it is time to get in touch with the Exchange Administrators
If you are an exchange admin yourself; below post would be helpful