Understand where mail storage comes from
The mailbox is an address and message collection, while stored message data draws on the account's object storage. This means one mailbox's messages can contribute to the same account storage usage as other data kept there. IaGenify does not provide a per-mailbox quota to compare or increase independently.
If mail storage is full, new attachments may be unavailable even though the message itself remains. Check the account's object storage usage and available capacity to understand the limit that applies. The mailbox count and the storage quota are separate account settings; creating more mailboxes will not resolve a full storage quota.
Free or add capacity in the shared storage
Review what occupies the account's object storage before deleting data. Removing an object is permanent, so confirm that it is no longer needed and keep a separate copy if retention matters. If the plan supports an object storage add-on, adding capacity applies to that storage rather than to an individual mailbox.
After freeing or increasing space, verify the mailbox behavior with a safe test message and attachment. A message that arrived without a stored attachment may not gain that attachment retroactively; ask the sender to resend it if needed. For account-specific limits, check the current usage report instead of relying on a generic mailbox-size estimate.