Should I use SPF spam protection or DNS blackhole lists for Server-wide mail settings

It seems that my Server-wide mail settings are preventing some legitimate email from being sent and received.

I have used both SPF spam protection and DNS blackhole lists (zen.spamhaus.org) for several years but lately I've lost a few clients because of persistent email issues.

Here is a screenshot of my current Server-wide mail settings in Plesk:

You can see DomainKeys spam protection and relay options are also set all of which were tweaked years ago to try to limit spamming through the server.

How do my settings look and what would you change?

Thanks in advance!