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Exchange 2013 MBX / CAS incoming email

quick overview of Email Server configuration:

Message Labs 3rd party spam filter scans and sends email through

VMware Virtualized on a blade server cluster connected to dedicated iSCSI storage.

All vmware drives are thick provisioned

2 CAS Servers Load balanced ~ 100gb C: (40% full)
2 MBX Servers with a DAG.  100gb C:  (50% full)  2TB D: (10% full) for mailbox databases and 500gb E: for transaction logs.   Logs get cleared after successful exchange aware backup.

My Question is this.

Backups started but ended up filling the C: on the mailbox database servers which obviously caused those MDB servers to stop working.   Since the CAS servers were still up and running, would inbound email queue up at the CAS Servers being unable to deliver it at all to a database server (i know its supposed to act simply as a proxy) or would it try to deliver them knowing the recipient does exist and hence the email is lost?  or would the end user get a bounce back, undeliverable?   Ive got a decent understanding of how the mail flow inbound works but just curious about this specific instance.   I had a 3 hour window,  just under 1 during normal business hours.
1 Solution
Simon Butler (Sembee)ConsultantCommented:
Mail should queue. At worst case scenario at Message Labs (as the hub transport rejects it). You shouldn't lose any email. This is just the same as the databases dismounting.

Really simple to test for yourself - create a new database, then create a new user.
Dismount the database and attempt to send an email via Telnet to the new user. You should see the message be delivered, then run get-queue and you will see the message listed.

Will SzymkowskiSenior Solution ArchitectCommented:
In Exchange 2013 Shadow Redundancy is the place where the mail items will queue before the mail item/s actually reaches the speified mailbox. This process is on the Mailbox server and has no bearing on the CAS role (as you stated CAS is basically a glorified Proxy which is stateless).

Shadow Redundancy is enabled by default unless otherwise disabled from Powershell.  Also as you have stated this will also be available on the second DAG member. The queues will hold on to the messages until they expire base on the number of days you specify.

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