Well, first of all you should be getting the contents of the submitted from from $[man]_POST[/man]; maybe you're already doing this and populating $header, $newmessage etc. from it, in which case the mangling might be happening at that point. That's all I can see from here, since the echo() is the first time that $newmessage is mentioned and you say it's already corrupt by that point.
Other observations:
You aren't quoting your attribute values, which is a common cause of form-submitted strings being truncated, but I don't see that being the cause here.
Are you properly escaping your data before embedding it into the SQL statement? If not, an apostrophe in $newmessage for example would cause all sorts of grief.
On an efficiency note, I notice that you update the message and then immediately retrieve the message from the database - even though you already have it. Is this necessary?