Comment 5 for bug 1031099

you can try:

Open the RTF file in a text editor, such as Notepad or TextEdit.
Check the file’s encoding settings to ensure that it is set to the correct encoding scheme. The most common encoding schemes for RTF files are “Windows-1252” and “UTF-8”.
If the encoding scheme is not set correctly, you can try changing it to “UTF-8” and saving the file.
After changing the encoding scheme, try converting the RTF file to MOBI format again using Calibre.