The callouts do not currently display automatically after a server post-back and will only work for custom validators which utilize client-side validation. Even after a post-back the callout will display when the form is re-validated when a postback is attempted again.

ValidatorCallout is an ASP.NET AJAX extender that enhances the functionality of existing ASP.NET validators. To use this control, add an input field and a validator control as you normally would. Then add the ValidatorCallout and set its TargetControlID property to reference the validator control.

The control above is initialized with this code. The italic properties are optional:

<ajaxToolkit:ValidatorCalloutExtender 
    runat="Server"
    ID="PNReqE"
    TargetControlID="PNReq" 
    Width="350px"
    HighlightCssClass="highlight" 
    WarningIconImageUrl="warning.gif"
    CloseImageUrl="close.gif" />

• TargetControlID - The ID of the Validator to extend
• Width - The width of the callout
• HighlightCssClass - A CssClass to apply to the invalid field
• WarningIconImageUrl - The path to a custom warning icon image
• CloseImageUrl - The path to a custom close image