The outgoing message from example.com to the external bt.com is rejected with the following error:
CONFIG_TEXT: 550 Message rejected for policy reasons (220.127.116.11)
BST - Please refine your SPF record for example.com using best
practice. Guide for bulk senders www.bt.com/bulksender (in reply to MAIL FROM command)
Incorrect SPF policy is set in SPF DNS record.
1. Log into Plesk.
2. Navigate to the tab Domains > example.com > DNS Settings, edit the SPF record and change "+all" value to "~all" or "-all".