You copied a series of users as a string from Outlook and pasted it into a User Control inside a SharePoint 2010 site.
You click on the "Check Names" icon and get the following error message:
"Item cannot be more than 256 characters."
For example, you insert a number of users that add up to less than 256 characters and click the "Check Names" control. You then add another batch of users and click "Check Names", and so on until you have added and resolved all of the desired users.
For more details, please check this MSDN article:
Article ID: 2654222 - Last Review: Jan 10, 2012 - Revision: 1