SPF allows you to authorise the sending sources for your outgoing emails. As a result, you'll be more protected against spam and email fraud. Follow these instructions to enable SPF on your Plesk server:
Login to your DNS registrar’s account
Navigate to Tools & Settings > DNS Template
You need to edit the DNS TXT record related to SPF
An SPF TXT record for Plesk will have the following syntax:
yourdomain.com TXT v=spf1 +a +mx +a:test.plesk.com -all
Publish this record in your DNS records section for your respective domain.
Note: If you don’t have an existing SPF record for your domain, simply publish the above-mentioned record to enable SPF for outgoing emails using Plesk. If you do have an existing SPF record in your DNS, you need to modify it to include Plesk.
Save changes
Wait for 24-48 hours to allow your DNS to process the changes
Login to your Plesk account
Go to Tools & Settings > Mail Server Settings > SPF spam protection
Enable the check box for SPF