all postfix mail forwarding rejected by my gmail account

I've been using postfix on a digitalocean droplet for years to forward mail from several domains on that server to my gmail address.

Now they're all being blocked by google, with this message:

Our system has detected that this message is 550-5.7.1 likely suspicious due to the very low reputation of the sending IP

running postfix 2.11. Can't upgrade that either; apparently ubuntu refuses to recognize postfix 3.

Can anyone help? I have no idea which way to turn.

How can I securely forward emails directed to my domains without running into this problem?