Email Thread on auto-reply

Question

I have a python script that fetches me the message and recipient email id, and then uses outlook to send mails to those email ids (registered on gmail). The sender email id is registered on another domain. Nonetheless, the script works. But the issue is while auto-reply is set on client ids, it is registered in inbox as unconnected to the original sent message thereby missing the whole purpose of the task wherein I need to have trail mail or thread mail. I am looking at possible solutions here. Can something be done on sender's side to enable the solution? Or do I have to link client emails with Zapier that allows me to include the body of original mail? I will really be grateful for your help. Thank You!

I have tried changing microsoft outlook settings (since I can only access sender's email id that is registered on a domain other than gmail), but to no avail. I tried looking for possible solutions online, but reached a dead end except handling client accounts (which run in 100s) and linking them with Zapier and setting it up.