This is probably because your house IP looks like a residential IP, which it IS. The problem with using a residential IP is that it looks like spam. Just use an IP address from a server room, like Google Cloud man. There is likely no reason to send from your house’s IP address. If you really need to involve your home in sending mail, SSH tunnel (or other proxy) your traffic so that it exits from a business IP address.
If you really want to do this (which I don’t recommend), what you want is to add your home IP address as a relay (I believe that is what it is called). Not really sure how you’d want to set it up, do you want to send mail from your house to the server, from the server to the outside world? Do you want to directly from your house to the outside world?
So if you want to send email through your server from your house, you want a relay (I believe). To add your home IP address as a relay, on your server edit this file: /etc/postfix/main.cf and play with the relay variables to add an IP address. I suggest Googling the Postfix manual to be sure about what you’re doing.
If you want to send mail directly from your home, add your home IP address to the MX record for the domain. If you go with Namecheap, I would suggest contacting support to be sure of details.