|OK, I'll start by saying that I am not an Outlook expert. Far from it!|
That said, the symptoms of your issue sound very similar to what happens to me when I use Outlook 2010 under Win 7. I'll describe the issue...perhaps that might point you in a direction that will help.
First some background:
If I create a new email, it is formatted in Rich Text by default. If I reply to a Rich Text email, the reply is formatted as Rich Text by default. In both cases, the automatic signature that is inserted into my emails is formatted as Rich Text. (portions are bold, my email address is a link, etc.)
However, if someone sends me a Plain Text email and I click reply, Outlook inserts my signature as Plain Text. If I enter text in the body section above my signature, it goes in as Plain Text.
Ok, now here comes the part that is similar to your issue:
When this problem first appeared, I would delete the Plain Text signature, use Format Text to convert the reply email to Rich Text and then manually insert my saved signature. While the signature would indeed be inserted as Rich Text, any text that I had already entered into the body would disappear. The only thing that would be in the note was my Rich Text signature and the original email from the sender.
In order to make it work, I have to delete the Plain Text signature, convert the reply to Rich Text, then insert my signature and then enter the body text above my Rich Text signature.
Could it be possible that somewhere in your process you are adding text to a Plain Text email which then gets converted to Rich Text before it gets sent? If an automatic signature or something else is added after that, the new text might get deleted as it does in my case.
I'll admit that this is a stretch and a guess, but there certainly seems to be a bug in Outlook 2010 related to converting between Plain Text and Rich Text and adding new content.
I hope that this points you in a direction that leads to a solution.
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