Just some feedback to benefit others with this very vexing problem.

The solution I found by trial and error after weeks of thrashing around more less matches the others here.

I moved from Windows XP to Windows 7 64-bit using PCMover and, because my trusty Outlook 2003 would no longer work with 64-bit I installed Outlook 2007. I started getting the problems mentioned here (Your message did not reach some or all of the intended recipients....etc. etc.). To compound my puzzlement, each time the error arose I discovered that Outlook had changed the account from which the message was sent.

I tried reinstalling Outlook 2007, deleting and reinstalling accounts, etc. etc. etc. and nothing worked. Then, each time the error came up I opened the adressee's file in Contacts and retyped the email address. This was a bit of a slog, of course, because if the message had been sent to 8 people, for example, I didn't know which address to change so I had to go through and change them all.

Anyway, the problem seems to have stopped but, as someone else said here, it is annoying that one doesn't know how it arose. It seems that when the Outlook addresses are transferred to the new installation in Windows 7 some of them become corrupted in a way that makes Outlook (a) change the Sender account and (b) throw up this error message.

I'm also very disappointed with Microsoft for not having a paper on this problem which, it seems, has been known - together with its solution - for quite some while.

Paddy