Old emails missing message body

I've just been looking through a few old emails and I noticed that many of 
them no longer have body. Looking at the source there is nothing below these 
lines:

Status: R
X-Status: N
X-KMail-EncryptionState:  
X-KMail-SignatureState:  
X-KMail-MDN-Sent:  

I do have back-ups of most of them, but is there any way of getting back the 
other ones? Also why is this happening?
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