Platforms like upwork have a feature where when you get a message through those platforms, you also get an email.
Instead of logging Upwork to reply, the user can simply reply to the incoming email, and the message is then automatically sent by upworks backend to the original person who sent the message.
How exactly is this done in a way that Upwork knows the inbox to send the message to, and is able to receive a message via email.
It seems Upwork handles incoming emails by detecting any project ID in the email message - it can be an ID in the subject or in the email body and etc. So, the email can be recognized easily and processed. Also additional information can be checked on the incoming email - the sender (whom the original emails was sent out), the recipient and etc. Actually, it is up to your needs how algorithm should be working.