Currently we are renting a virtual serving hosting services for our e-mail as follows:
Storage: 10GB
Hard disk: RAID 5
Dedicated RAM: 512MB
Network Traffic (per month): 500GB

Inside the server, followings are installed:
CentOS: 512MB RMB
Zimbra: 3GB (1G for evaluation and testing, 2GB for production)
E-mail accounts: Total 1400M

Zimbra turns down or turns to off-line every two week and we cannot receive and send e-mails.
Our IT friend explained the problem is occured due to insufficient capacity, upgrade to at least 20GB is the only solution to this problem.

Would like to know if it is the solution? Any more advice?

Thank you.