Been running ex 2013 cu1 since early April. No major problems. Current db setup is 3 db's in 1 dag spread over 2 servers.  Server 1 is local and holds primary copies of db's.  Server 2 is remote. Suddenly users stop receiving mail, sending mail gets stuck in outbox.  Primary db was  mounted,healthy. I activated secondary copies of db's on remote server and mail starts flowing again. Both copies of db's appear healthy.  If you switch back mail stops again.  Any ideas?  Thanks in advance.

Is there anything in the event log?  If not, the next step I'd start taking is to turn up event logging.  Don't ask me where--that is left as an exercise to the reader.