Can two email accounts have the same IP? If so does this give the two parties access to each other's email account? Does it allow them to block mail to each other's account?
Yes and no.
Yes 2 emails can have the same IP. It's actually not the email accounts that are assigned the IP but the computer that those emails were sent from. So in other words, if I have 10 email accounts set up on my computer, then all 10 would originate from the same IP.
If a public computer were used by several different people to check their hotmail accounts, then the originating IP for the accounts checked would all come from the same IP.
When you block an email address, you're actually blocking the actual address [email][email protected][/email] and not the IP address the email came from.
No to the portion of your question about giving each user access to each others accounts. If you login to your hotmail account from your computer and then someone else comes along on your computer and logs in to their hotmail account, they can't see your emails and you can't see theirs.
Hope that answers your questions.