What this means in practice
If storage is full or a part is too large, the message can still arrive without that part’s link.
When an attachment is missing, check both available storage and file size before retrying.
Practical guidance
Attachments use the account's object storage rather than a separate mailbox quota. A stored attachment is accessed through a link that expires after fifteen minutes; the reader can renew the link when needed. Because mail and other objects share one storage allowance, attachment capacity should be considered alongside files and any other stored content.
If an attachment cannot be stored because the quota is full or the part is too large, the message itself can still arrive; that part simply has no storage link. When an expected attachment is missing, check whether storage capacity or the file's size prevented it from being saved. For ongoing mail use, monitor the shared storage total and remove or archive objects you no longer need. This explains why an email can be present even when one of its attachments is not accessible.