Email message sent from a domain in Plesk is rejected: 550 Message rejected for policy reasons

Symptoms

Outgoing messages from Plesk domain example.com to an external one example.net are rejected with the following error:

550 Message rejected for policy reasons (1.1.1.2)
 BST - Please refine your SPF record for example.com using best 
 practice. Guide for bulk senders www.example.net/bulksender (in reply to MAIL FROM command)

Cause

Incorrect SPF policy is set in SPF DNS record.

Resolution

1. Log into Plesk.

2. Go to the tab Domains > example.com > DNS Settings

3. Edit the SPF record and change “+all” value to “~all” or “-all“.
   OR
   Replace the SPF record value with default one:

   v=spf1 +a +mx +a:example.com -all