I've read a lot of threads where others are talking about Zimbra server performance on ESX/ESXi, but my experience tells me it is confusing.

For example, I have one Zimbra 8.0.2 server on Ubuntu 10.04, running on ESXi 5.0.
ZCS has 8 GB dedicated RAM, and 2GHz reserved and available all 10GHz total power of host, which runs on 2 x Quad XEON 2,5 GHz.
It is single-server all-in-one Zimbra installation, with 400 mailboxes.

Average MTA digestion is approx. 9.000 email messages per day, except a spike of 10.000 up to 100.000 mails sent out by some mailing outburst once per day or two.

Average CPU usage is 7%...
...but during these mailings, where single user sends mail out to 10.000 or sometimes even to 100.000 users, CPU of total host burns oil at 80-100% and chokes the whole ESXi server.

Is such a behavior normal?
Well, I know the mailing application does NOT send 1 single mail with 100.000 recipients, but rather sends out 1-by-1, single mail at the time, so there is really huge traffic. But should that choke my whole ESXi? It becomes unresponsive, mail queues extend to 1000 or more mails in Queue, mail delay is up to 4 hours, and web sites, which are also run at this ESXi, return "Timeout".

What hardware upgrade would help most?
Is it RAM upgrade?
Or should Ultra fast SSD PCI-e adapter ioDrive from Fusion boost performance?
Or should I replace SATA disks in RAID 10 with faster SAS 15k disks?

Ideas welcome.