When sending an email you may experience the following bounceback message:
550 5.7.1 H:DBL [126.96.36.199] Connections originating from
dynamically assigned IP addresses are not allowed. Please use your
ISP's relay servers.
This type of rejection happens when the remote server detects your mailservers IP address as being a dynamic IP instead of a static IP of a mailserver. This can happen if you are trying to send email from a server hosted on your home network, but it can also be a false-positive detection on the remote mailserver side, mislabeling your server's IP as being part of a public network.
If you are sending from a home network, your IP address is blocked from mail delivery on that remote system to help prevent spam from entering their network. In this case, you would need to follow the instructions in the bounceback and setup a relay to send messages to that particluar user.
If you are sending from a server and getting this bounceback error, make sure your main IP address you are using to send email has correct reverse DNS. If you have that in place and you are still receiving this error, the problem is with the remote mailserver you are trying to send the message to. You will need to contact that mail provider through another account and let them know about this issue as there isn't anything you can do on the server side to fix this problem.