What is the correct usage between "a chain of email" and "a chain of emails"? Why?
By "a chain of email" I mean an email thread where many people have chimed in from time to time and that has grown very large over time.
An email thread is an email message and a running list of all of the subsequent replies pertaining to the original email.
Getting back to your question, mail can be considered a mass noun (much like sand, rice, and money), meaning that it can be tricky figuring out when and when not to pluralize the word.
If, for some reason, you didn't want to use email thread, I would call it a chain of email if it was one message that got forwarded many times. To me, emails connotes several distinct messages from several different sources, not one message that got appended with several subsequent replies.
Email thread, email chain and email stream can all mean the same thing: a collection of email messages including the original and all subsequent replies and forwards from the original sender, recipients and anyone else who was copied or blind copied.
Such a collection of messages usually maintains the same Subject, but this is not a requirement. Although not recommended, it is usually possible to change the Subject before replying or forwarding.
Please note, the email thread/chain/stream is a collection of the actual messages - not simply a list of the messages.