WebMail Lite allows you to send and receive mails using SMTP and IMAP protocols. However, when it comes to sending and receiving mails, there are things which are done without direct user interaction: Message filtering Depending on certain conditions, incoming mails need to be moved to a different folder, deleted from server or processed in some other way. Forwarding All incoming mails must be re-sent to another email address. Autoresponder Whenever new mails arrive into the account, sender must get an automated notification of some kind.
One common thing about the above items is that WebMail Lite cannot do any of those. Action should be performed regardless of whether you're logged into WebMail or not. In fact, mailserver itself needs to perform those actions, but how do we ask it to do that?
That's where Sieve comes into play. In fact, Sieve is a scripting language, and it allows for telling mailserver how it should process mails. Naturally, mailserver needs to have Sieve support, but it's not enough. You will also need ManageSieve service installed on mailserver. Yes, that's right, setting it up on a separate webmail box won't work.
Technically, ManageSieve is a thing somewhat close to FTP server: you cannot upload files to webserver directly, as you don't have access to its filesystem, so you need an intermediary. Similarly, ManageSieve service is a software which lets email client and mail server talk to each other.
There's a good chance that your server already has both Sieve support and ManageSieve service installed, and you'll need to check that first. If the first condition isn't met, this might get very complicated, up to the level where recompiling mailserver is required. As for ManageSieve, it can be installed and configured relatively simply. For example, there is Pigeonhole agent created for Dovecot mail server. Bear in mind though, we're speaking of mailserver software here, so it's assumed you're pretty good at what you're doing.
In the ideal case, i.e. when Sieve support is available and ManageSieve server is up and running, all you need to do is to enable Sieve support in WebMail.
Check Use Sieve for filtering documentation page for detailed information on configuring Sieve support in WebMail Lite.