I have this huge dumbness I need to clear up. I am not sure under what circumstances (all?) IP addresses can be seen.
When you send an email from a web based email service such as hotmail, gmail? Can it be seen by the email recipient?
When you post to a blog in the comments section? Can it be seen by the blog owner?
When you post on a forum (I realize forums may vary), can the forum owner/admin etc.. see your IP address?
I work at a web company and we are able to view all of our users IP addresses.
Also when you post to a blog, yes the webmaster or whoever approves the comments can see it.
You can use IP checkers after to find out the exact location of a user.
As for email recipients, no I don't believe that is revealed. However, the host (Gmail, etc.) probably has that data.