Thank you for the provided information.
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Depending on the source, the pasted content is imported with different format providers and the issue is related to the RtfFormatProvider, which is used by default when pasting content from MS Word and MS Outlook. When pasting from a browser, the clipboard passes the content as HTML thus, the HtmlFormatProvider is used.
As a workaround, you could try changing the ClipboardHandlers of RadRichTextBox as described in this help article
Hope this helps.
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