This issue was opened retrospectively. Some clients using our anti-spam service may be noticing delayed delivery of incoming emails from external senders.
This issue is caused due to the external mail providers NS records for our domain being incorrect. They have failed to update the NS records for our domain according to our TTL schedule.
This occurred due to a brief outage of our NS records. Compliant mail systems updated their records by 6 pm yesterday, 24/10/18.
Unfortunately there is nothing we can do to mitigate or resolve this issue. We expect all providers will have updated their records on or before 5 pm, Saturday 27/10/18.
At or before that time, external senders email will arrive.