Plain text messages are aligned 'center' when received, or replied to.
New HTML emails start as 'center' aligned.
I can recreate this behaviour by
1. creating a new message in OL2007,
2. focusing the cursor in the message body area by clicking in it or
tabbing to it,
3. selecting the "Format Text" tab from the ribbon in the message,
4. opening the styles pane by clicking the expand button in the lower
right corner of the "Styles" ribbon box,
5. right-clicking on the current style ("Normal," in my case), and
choosing "Modify" from the context menu,
6. clicking the "Format" button in the lower left of the "Modify
Style" dialog and selecting "Paragraph..." from the sub-menu,
7. changing "Alignment" listbox selection to "Centered" and clicking
"OK,"
8. selecting the "New documents based on this template" radio button
near the bottom instead of "Only in this document,"
9. clicking OK, and
10. closing the new message (without saving changes -- this doesn't
matter).
All new messages now start with the message body aligned center.
Theoretically, then, if you follow this procedure, but change the
alignment box from "Centered" to "Left," you should get the desired
results.
I am not entirely sure this is going to work for you, though, because
when I tried to reproduce the problem with the steps above, I did not
see a change in the way plain-text messages were displayed.
Hope this helps, though.