Add this rule to affected folders,   run the rule & remove it.    
 (turn off other rules you don't want running when you do this!)     
Code:
++:!+: replace(gfv("E-Mail"), "\x0D(?!\x0A)","",1): 
        In this example,  the "E-Mail" folder is used.  The rule should be placed in the E-Mail folder !!       
 a 'better' rule is:     
Code:
++:!+: replace(gfv("E-Mail"), "\x0D(?!\x0A)","",1): [E-Mail] ~= "\x0D(?!\x0A)" 
        How this works ?   
 The 'hidden line break' is a hex 0D,   
 0x0D followed by 0x0A is a NOT HIDDEN LINE FEED   
 so,   
 we want to kill ONLY 0x0D by itself.     
 Slang rule allows touching HEX VALUES DIRECTLY.  (which is pretty cool).     
 hope helps!