Are email servers like Gmail, etc. able to capture a computer's serial number on sent emails? If so, are there ways to block that type of sensitive identifiable information from being captured by email servers. Thanks
No, email servers (and web sites) are not able to capture your computer's serial number. However, there are unique identifiers that can be used to "fingerprint" your browser and uniquely identify you.
There are several mechanisms to do this, most of which can be embedded (sometimes silently) to any web page you visit. Things like local system time (provided client-side), list of fonts installed, browser resolution, and even graphics-rendering fingerprints can be used to uniquely identify your browser.
The EFF has a proof-of-concept called Panopticlick that demonstrates this functionality. When you use this service, you can see whether Panopticlick has seen your specific browser "fingerprint" before. If you're unique, you can theoretically be uniquely tracked.
Howevver, to answer the specific quesiton you asked: no, no one can see your serial number or MAC address on the Internet.