I think it would be great to have a IsMultilineAllowed property for this control, as AcceptsReturn property is only designed to ignore the return key entered by the user.

Here is my post in the forum:
http://www.telerik.com/forums/paste-multi-line-text-when-acceptsreturn-set-to-false
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Reason for Decline: The desired request is custom logic that could be implemented by inheriting the RadMaskedInput control and overriding HandlePaste.