Cannot add to the server Junk E-mail Lists, you are over the size allowed on the server. The Junk E-mail Filter on the server will be disabled until your Junk E-mail Lists have been reduced to the size allowed by the server

Microsoft Exchange has a limit to the size of the rule where your different "junk" lists are stored. By default, the limit is 510 KB.

In relation to this limit, there are four known causes for the above errors.

• You have a large number of entries in your Safe Senders, Blocked Senders, and Safe Recipients lists
  
  By default, you have a cumulative limit of 510 KB for all of your junk e-mail lists. If you have a large number of entries in these lists (cumulatively), then you may receive the above error when you attempt to add more items to the lists.
• The Max Extended Rule Size registry value is incorrectly configured on the Exchange server
  
  On an Exchange server, you can configure the following registry data:
  
  Registry key: HKLM\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
  Value: Max Extended Rule Size
  Type: DWORD
  Data: an integer specifying the maximum size, in bytes, you want to allow for the junk e-mail rule
  
  The value you enter in the registry is interpreted as the maximum rule size in bytes. Therefore, if you want to specify a limit that is higher than the default 510 KB, you need to enter a value higher than 522240 (decimal).
  
  The above error can occur when you inadvertently specify a value lower than the default limt. For example, you enter a value of 1024 under the assumption that this is doubling the default limit of 510 KB. Since the value you enter is interpreted as a number of bytes, a value of 1024 means you have configured a limit of 1024 bytes, substantially lower than the default limit of 510 KB.
• The PR_RULE_MSG_STATE property of the Junk E-mail Rule message is incorrectly configured
  
  With an Exchange mailbox, your junk e-mail settings are stored in a hidden message in the associated contents table of your Inbox folder. The subject of this hidden message is Junk E-mail Rule. This message has a property calledPR_RULE_MSG_STATE and the default value on this property is 49 (decimal) or 0x31 (hexadecimal). If you then disable junk email filtering in Outlook Web App, the value of this property becomes 48 (decimal) or 0x30 (hexadecimal). Therefore, if the value specified on this property is something other than 48 or 49 (decimal), then you may receive the above error.
• The "Also trust e-mail from my Contacts" option is enabled in Outlook
  
  On the Safe Senders tab of the Junk E-mail Options dialog box you can configure the Also trust e-mail from my Contacts option. If enabled, and you have a large number of contacts in your Contacts folder, you may inadvertently exceed the 510 KB limit on your mailbox. With this option enabled, Outlook attempts to add all of the email addresses for your contacts to the Safe Senders list. 

http://support.microsoft.com/kb/2669081