Blocked Senders size limit

When you connect to an Exchange Server, the Blocked Senders, Safe Senders and Safe Recipient lists are stored on the Exchange server so that these lists travel with you when using your mailbox on another computer or when using OWA. By default, Exchange will allow you store up to 1024 addresses in these lists.

Don’t add every junk address as blocked

When you reach this limit, you might want to reconsider your spam fighting strategy. In general, your Blocked Items should be quite empty. There really is no need to add the address of a Junk E-mail that ended up in your Inbox to the Blocked Senders list. Usually these addresses are spoofed or auto generated and are only used once. You should only add addresses to the Blocked Senders list when you receive multiple emails from that addresses for an extended period of time and when there is no other way to get rid of this mail stream (legitimate newsletters should have an unsubscribe option which are usually placed at the bottom of the newsletter).

Delete or move instead of marking as junk

Instead of using the Junk E-mail “right click” or Actions menu to move the message to your Junk E-mail folder, consider using the Delete button. If you really want it to go to the Junk E-mail folder instead of the Deleted Items folder, move the message there instead via the Move To option or via drag and drop. This way, the address won’t be added to your Blocked Senders.

Note: Even though the mentioned error only applies to Exchange users, the above strategy still applies when you use an IMAP or POP3 account as well. For those account types, the added benefit of having clean blocked/safe lists is that receiving email is processed more quickly since the incoming emails will not have to be checked against a long lists of addresses.