What is the difference between Declared nb of mailboxes and Nb mbx used?

Short answer

Don’t worry about it.

Long answer

Declared nb of mailboxes is used for billing purposes. This number is a record of how many users are configured on the service.

This is a number that is given to us by you when you sign up and is updated once a year. You can contact us to request an update if you have at least a 10% change to your number of users.

Nb mbx used is a number generated by Cumulus based on the number of email addresses we have been able to deliver emails to. It’s meant to be a helpful guide used during your yearly mailbox update to give you a sense of how many users are on your domain. This information is not used for billing.

Since Nb mbx used counts email addresses, it is normal for this number to be 2-4 times larger than Declared nb of mailboxes (depending on the average number of aliases each user has). If the Nb mbx used is any larger than this, it might be a sign that your mail server is not rejecting invalid email addresses during the SMTP Transaction. If so, please make sure to perform recipient validation during the SMTP Transaction.