The "problem" is that smtplib.SMTP does the following, in essence:

1. Open connection to server
2. Send EHLO
3. Read server response

The PROXY injection causes a response from the server, but the response gets "cached" in the incoming TCP buffer. The injection script likely does not consume this buffer, resulting in Step 3 above reading this "cached" response. 

So from the point of view of smtplib.SMTP, the "503" message arrives _after_ EHLO is sent, while in actuality the "503" arrives before EHLO is sent.

I suggest rewriting the port forwarding script so that it consumes the response from the server first before transferring to smtplib.SMTP