The British Telecom (BT) team informed us that these recent deferrals are occurring due to a high rate of mail sent through our service from customer domains lacking SPF records, and should only affect messages from sending domains without any SPF record published to their DNS.
An SPF record is an entry established in a domain’s DNS which includes a list of any sending systems which are authorized to send mail on behalf of that domain. When a mail server receives a message from a sending address at that domain, it can independently compare the source of that message to the sending domain’s SPF record to verify that it is not spoofed.
While SPF records are not required by internet specifications, as we have no control over the policies established by external recipient systems like British Telecom, we would advise any MSP Mail customer impacted by these deferrals to ensure that there are SPF records in place for their domain(s) which include both our sending IP addresses, and their own mail server’s.
You can find more information about establishing an SPF record here: https://support.solarwindsmsp.com/kb/msp_mail/How-do-you-setup-an-SPF-record-for-a-domain
If you already have correct SPF records published for your MSP Mail sending domain, but are still seeing these deferrals from BT, the BT-hosted recipients of those messages should send the details of those rejected messages to British Telecom’s technical support.
Please contact our technical support team with any questions, or if you need assistance setting up an SPF record.