This took a long time to work out. As usual, someone had walked the path before me but it took a while to find that solution amongst all the false leads (in my case).
Our company had recently switched it’s email from Exchange Server to Gmail. (Yes, they are aware of the security issues. Anyway …). Not liking the GMail interface etc I wanted to continue using Outlook for my mail client and using GMail’s IMAP to do that. After setting that up (lots of posts on the net on how to do that), I noticed the following problem.
Important note: See this post BEFORE you delete your old Outlook Exchange file. If you do it the wrong way you will lose all your email!
The problem: When I created a new email to some of my contacts, or replied to an email from one of those same contacts, the email stayed in the Outbox of my default PST file. Sometimes it threw the error 0×80040201. Nothing I did would cause Outlook to send the email. Other email addresses worked fine! Very weird. Eventually I discovered the post below that explained that the problem was due to the Outlook.nk2 file that contains the data for the auto-complete functionality of Outlook. That file remembers whether the address is an Exchange Server address or an SMTP address. And guess what? All the addresses I had problems with had Exchange settings.
So, see the LAST item on this post: http://www.pcreview.co.uk/forums/getting-error-0×80040201-t1872774.html which explains a couple of fixes.
Alternatively, download this: http://www.nirsoft.net/utils/outlook_nk2_edit.html and delete all the email items with an account type of EX (as opposed to SMTP) and follow the instructions. You will then need to type the full email address the next time you send to that person and it will then remember the correct settings. Some addresses had both EX and SMTP entries so deleting the EX entries allowed the SMTP entries to correctly auto-correct.
I hope this saves someone the hours it took for me to find the solution.
NB: there is still a problem with Outlook that appears to be a bug. To get the email to actually send, you must click Send/Receive / Send ALL. I haven’t found a way for it to send automatically but at least I can send email with a couple of extra button clicks!
Now, I wonder how well Thunderbird’s IMAP works?