3. is the reason why NGinX is the recommended proxy in front of webapps with scarce parallelism (for example Ruby with Unicorn; see http://unicorn.bogomips.org/PHILOSOPHY.html for an explanation) when "slow clients" are to be expected. NGinX is protecting the webapp from blocked workers by slow clients and Outlook Webmail seems to behave just like one. I don't know by heart how to tune this behavior if one wants to avoid it but this property is the main reason we use NGinX.
This sounds like something else. In the outlook case their servers they seem to use the connection as a stream (which is actually valid, although not really supported by browsers outside of the event-stream class), where the server only writes little chunks of data of a time. But the server there not hindered from writing by a slow client - it simply has not more data to write at that point of time.
