The shadow mailbox contains messages for recipients who have full mailboxes. ELA cannot deliver these messages until the recipients make space in their mailboxes. The shadow mailbox can hold a large (but fixed) number of messages. It may also become full if the messages in the shadow mailbox exceed a certain size. If the shadow mailbox becomes full, no more messages can be delivered.
The messaging application can take up to 2 weeks to determine that a message is undeliverable and to generate a log entry for a delivery failure.
Find out whether persons with full mailboxes have abandoned their mailboxes. Abandoned mailboxes routinely consist of unwanted messages distributed by often-used lists. You can then remove these mailboxes, or ask subscribers with full mailboxes to delete at least half of their messages.
If you get this alarm regularly, evaluate how your business uses ELA: