take the mms.apk from the inverted add-ons and replace the one in \system\app. You can rename the current one to mms.apk.bak and leave it there in case you want it back or have problems with the inverted version. You can use something like 7-zip on a Windows PC or a root browser-like program directly on your phone to browse the addon package to get the inverted apk file you're looking for.
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Thank you for your suggestion.
That was the first thing I tried to do after I was told that there is an inverted addon.
The funny part is, that there is no MMS.APK in the zip.
Am I missing something? This is the file I downloaded: "Helly_Bean_Inverted_AddOn_20120729.zip"