What happens to incoming mail when a mailbox is over quota and has forwarding set?
Incoming mail is forwarded to the specified forwarding address. This is the correct behavior for mailbox forwards; mailbox forwarding addresses are treated as independent recipients of the message.
If keep copy is specified in the Fwd Set command, the forwarding address is added to the "recipients" list in the message envelope.
If keep copy is not specified, the forwarding address replaces the original recipient in the message envelope. The message is then independently delivered to each recipient, and the over quota status of one recipient does not affect delivery to any other recipients.