This is all making sense then. I am kind of an idiot because I should have realized it from the header.
It's not anything on your computer causing the problem. The problem in this case is that the sending IP address is "blacklisted". And when naive technical people start making decisions, they're usually idiodic and based on heresay vs. facts. The morons at Comcast use what's called a RTBL or "RealTime Blacklist Lookup", which checks the databases of extortionist enterprises like SORBS (Spam and Open Relay Blocking System), who "blacklist" an IP address when/if a single user reports that they have received SPAM from it. Since you (like almost everyone else) share an sending server with many other people, if one of those people has sent spam and it has resulted in being reported, the server is blocked by any receiving entity that checks email against a RTBL until it is de-listed.
I quick check of the IP address of the comcast server sending the message ( qmta12.westchester.pa.mail.comcast.net [188.8.131.52]) reveals that it is, in fact, blacklisted with SORBS.
|SORBS-SPAM || LISTED ||DetailReturn codes were: 127.0.0.6 ||2859 ||16|
The reason this is only hapenning when you send from Outlook is that doing so uses your local outgoing server rather than the main servers that the web mail uses. These servers are not monitored closely by comcast, and are not actively de-listed when they appear on a list.
Ultimately, the fault in this case is Comcast. It is their responsibility to keep their servers from appearing on blacklists. They are also the idiots that are using the blacklist service to block their own email server.
That's a lot of info. I can explain more detail if you have questions.