Email threading in Gmail
You can use your Gmail account as an email threading tool. Two emails from another email account, one received in the inbox, and a second replying to the original email, when forwarded separately to a Gmail account, will be automatically threaded together.
Gmail will thread all emails that:
1. Have the same subject, recipients, and senders.
2. Were sent within one week of one another.
3. Have a reference header ID in common.