I use Zimbra Release 7.1.3_GA_3346.UBUNTU10_64 UBUNTU10_64 FOSS edition
on VMWare ESX 5.0 (16 GB Memory, 8 CPU on Zimbra host), over 3000 mail's users.
The Zimbra server worked great for a long time, until one terrible day mailbox service die for no apparent reason.
Logs showed the presence of errors:
"java.lang.OutOfMemoryError: Java heap space"
before stopping the service. Help me please.

P.S.
mailboxd_java_heap_size=1996
zimbra_zmjava_options=-Xmx256m
mailboxd_java_heap_size=25