I've been having a problem for (I'm guessing) a few months that I finally resolved and I'm posting it here as a warning. The problem occurs in Outlook 2003 but may also occur in later versions.
I'd start composing an email in Outlook 2003 and save it to the Drafts folder. I'd return to it some time later, make a few changes, then click Send. I only noticed a problem when someone replied and included the original message which was not what I sent, but instead the text from when I saved the first draft.
After some experimenting I determined that after doing the first save, any other changes were discarded, even if I did another save. Even if I made changes and kept the email open, what was sent was the original draft.
After a lot of searching I determined that the problem was caused when Internet Explorer was upgraded to version 11.
To date, Microsoft has not supplied a fix, however, by telling Outlook to use Word for editing emails rather than the native Outlook editor I was able to eliminate this annoying problem.