Hello,
You need to cancel the postback, let the spellcheck work and initiate the postback again when it finishes. Otherwise, the postback will dispose the spell because it works on the client.
There are various ways to do this and perhaps this KB article will help you with one approach: http://www.telerik.com/support/kb/aspnet-ajax/spell/details/invoke-separate-spell-check-for-each-textbox-and-submit-the-form-on-success.
Another suggestion is to use the client-side API and events of the control:
For your convenience I prepared a small example here:
Regards,
Marin Bratanov
Telerik
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