PHP: 5.4 and above
RainLoop webmail was designed with efficient memory use in mind, so it can work fine even on low-end web servers.
Regardless of mailbox size and number of mails in it, memory use per active user is always kept at a minimum, even when very large mails are processed.
In most cases, application speed directly depends on mailserver performance and network bandwidth available.
In addition, built-in caching system allows for improving overall performance and reducing load on web and mail servers.