[slowbdotro]

If volume low, a cheap option would be to use Postmark (buy credits not their subscription). I have nothing but good things to say about them.

Otherwise, as you are already self hosting, getting a new box in a different datacentre (and provider) and use that for relaying could be an option. A tiny 1cpu, 512mb with an IP whitelist to your main server wouldnt set you back much. I bought a 2cpu 1GB for $40 annually.