I have a user getting sporadic errors in Outlook 2007. I have Exchange 2010.
It has happened to internal and external recipients.
If I delete the text from all previous emails in my reply, then it will send just fine.
This user has no issues in termserv or in OWA.
I have put them in cached and non-cached mode and have recreated their OST.
The error that pops up is "cannot send"