It all depends on the amount of traffic on your system, as well as the size of your mailboxes.
RAID5 has poor write performance, and when a message arrives there are a lot of writes.
At a minimum:
- the whole message gets written to disk
- its content/headers/attachments gets anaylzed and indexed, which is multiple writes
- rows are inserted/updated in MySQL, as well as data written to the MySQL transaction log
- Zimbra has its own application-level journal file for crash recovery
- there are other log files written to (log4j server-level logging)
If you aren't having any noticable performance issues, you should be ok.
In general though, we recommend against RAID5 because in our testing we have seen very bad performance under it, to the point where a moderately loaded system will slow down to a crawl with RAID5, but have plenty of headroom on RAID1+0.