1. Within a business structure, IP addresses for client machines are never "seen" on the internet as they are given special addresses (ie: 10.x.x.x and/or 192.168.x.x) which are specifically reserved for this purpose. As such, if you emailed me, my internal address really won't have any bearing.
2. On the public side of things, hotmail (for example) may have multiple physical servers on multiple networks - this helps spread the usage and traffic to multiple computers to prevent any one given machine from being too busy. These servers can be on any different A, B, or C class of IP addresses any may bounce (forward) mail through other approved mail services to get the message from it's source onto it's intended destination.
3. Internet AND Private addresses are normally given out by what's called a DHCP server. For computers who are running 24 hours a day, the IP Address given to the machine rarely changes. For machines which don't get turned on but every day or few days, these IP addresses are more likely to change as the previous one it had has already expired from being inactive.
4. Email Proxy Servers... when getting email sent from various systems and the X- records look like they're coming from all over the place, see item #3 above. In some instances, if you check one of the IP addresses and it's found to be a PROXY, it may be wise to block the email as it's more apt to be spam... or worse. Last I knew, MS/hotmail doesn't use proxies. Using the command line from your computer, type in the following:
nslookup ip_address
If there's a "name" online for the IP address, it'll likely show up here. If it's not hotmail (live.com or microsoft.com) related... it's not likely a valid email and unlikely from hotmail.
Example: nslookup 65.55.72.167
Hope this helps!