I installed Exchange 2013 CU8 and now everything seems to work fine except I have NO mail flow! Mailboxes can be access by ActiveSync, Outlook and on the OWA website. Mail appears to send without any errors but messages are not going out and they are not being received either. I send messages internally and they're never delivered nor are messages sent to myself. Can anyone shed any light on this? Thanks in advance!

Have you checked the mail queues and the transport service?  Did you put the server into maintenance mode before you started the updates?

http://blogs.technet.com/b/nawar/archive/2014/03/30/exchange-2013-maintenance-mode.aspx

Last night i just installed CU8 myself.  I ran into the same problem you did with the outgoing SMTP server.

Start or restart the following services by opening up the services console Start --> Run --> services.msc

 - Microsoft Exchange Frontend Transport

 - Microsoft Exchange Transport

In my case the Frontend Transport had not started after the update.

Hi,

On the basis of your post, I understand that mail flow cannot work, the message cannot be delivered and no NDR return.
If I misunderstand your concern, please do not hesitate to let me know.

Firstly, we need to check the related services about transport as tbswen mentioned.
Secondly, we can use queue viewer, message tracking log and protocol log to get details about what happens to messages as they flow through the transport pipeline. More details about Mail flow in Exchange 2013 and Transport log, for you reference:
https://technet.microsoft.com/en-us/library/aa996349(v=exchg.150).aspx
https://technet.microsoft.com/en-us/library/dd302434(v=exchg.150).aspx

Finally, please run command to double check the state of Exchange system component: Get-ServerComponentstate -Identity servername
If the state is Inactive, please run below command to active relevant component:
Set-ServerComponentState <Identity> -Component component name -Requester HealthAPI -State Active

More details about Server Component States in Exchange 2013, please refer to:
http://blogs.technet.com/b/exchange/archive/2013/09/26/server-component-states-in-exchange-2013.aspx

Thanks