(This next link includes a slightly more detailed version of the M$ one above).
I found all three above here:
I have had the similar situation in the past; found it sometimes to be a problem with my outbound server; sometimes with the isp. In both cases it appeared to resolve itself - though no idea why...
Also have re-installed Outlook to resolve it too on occasion; after first saving the .pst files elsewhere... (exported, and also backed up - (copied as such) to somewhere safe as nice to have two versions of safety...)