When i was checking email message's header in Windows Mail Live from GMail account i saw the following:
domain of transitioning does not designate "insert ip address here" as permitted sender
What does that mean? Did sender spoof his ip address?
Email servers can, by original design, send to/receive from any other server; remember, it was thought early on that there would be relatively few servers and people would just use terminals. As the number of servers grew, so did the "bad guys" sending spam.
Some servers now have a list of servers/addresses from which they'll accept mail - not individuals but servers. Don't think of the individual sending out a few dozen ads a day, this is designed to stop traffic from servers that are known to be friendly to those who dump thousands/hour. It's like the Post Office only accepting trucks from other official Post Offices at the loading dock. The idea is that, if I don't trust the guy "upstream" from me I won't take his packages and pass them along.
Probably not. Most ISP's do allow their own customers to send email using the email address of the ISP. However, this is restricted to their own IP address range.
Example: Say, my ISP has address range 10.x.x.x, and domain "example.com". They therefore tell other mailservers "hey, it's OK to accept emails from people with an @example.com address, as long as they've got an IP address in the 10.x.x.x range".
Now imagine what happens if you've got a laptop with WiFi connection. You might pick it up and walk off, and end up sending an email from somewhere else. Suddenly, your IP address no longer is that of your ISP, but somebody else's, yet you're still using the @example.com email address. This would generate the warning you'd see.
The reason you don't see this very often? Most people have their mail program configured so that their emails first go to the mailserver of the ISP itself. And most ISP's correctly list their own servers. It seems that in this case, either GMail made a little mistake, and forgot to put their own server on the "permitted sender" list, or someone else pretended to be GMail.