Emails to a former customer of Email Security.Cloud are failing delivery even though their MX record and other public information is correctly published.
When two Email.Cloud customers on the same cluster email each other, we use an internal DNS lookup in order to know where to deliver the emails. This is detected by the recipient domain and the inbound routes on that account, which are then used to deliver the email.
If a customer leaves our service but does not remove their routes/domains or properly cancel out the account, then we will still use their configured routes left in the accounts to deliver emails from other customers.
To correct the issue the former customer will need to either log in to their old account and remove their domains and inbound delivery routes, or they can reach out to our cancellations team to have the account shut down.
After that is done our system will use an external DNS lookup to decide delivery on the emails.
Please note internal mail delivery can also fail by to Address Registration due to the former customer not maintaining their active user list.