sometime email body goes blank only remain signature

[gfpoow]

X-Mailer: Zimbra 5.0.6_GA_2313.SuSEES10 (zclient/5.0.6_GA_2313.SuSEES10)
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit

sometime email body goes blank only remain signature
also when i check at sent folder the HTML tag not be converted as text

i sent same email again and it display normally (html tag converted and content appeared)

why?