Many mail clients have ways in which to setup rules for processing e-mail. Unfortunately, rules setup in other mail clients only work with that specific mail client, and only when the program is running. For example, setting up a rule in Thunderbird on yoru laptop to sort all mail from "email@example.com" into the "GmailUser" folder doesn't work when Thunderbird isn't open and you're on the road reading your e-mail on your iPhone.
Roundcube filters are special, in that they tell the server how to process your mail utilizing a language called SIEVE (which you don't have to know, Roundcube knows it for you). This means that, regardless of which client you use, a filter created in Roundcube affects each and every mail that is delivered to you, regardless of the client you use to read your email. The filtering process occurs while an email is being delivered. Filters do not affect mail that has already been delivered.
Filters are processed in the order you see in the list of filters. This also means that a later filter may not only do additional actions for a given mail, but may also undo the action defined in a previous filter. Be aware of this when making your filters.
To setup a filter:
Several "Matching Rules" and "Filter Actions" have additional help which can be identified by the blue circle with a question mark. Clicking on that icon will reveal additional help.
An example filter has been included in the "Example Filters" section of the filters page. It is HIGHLY recommended that you use the example filter and then alter the fields under the "Filter Actions section. The example contains the correct matching rules to ensure your vacation message does not respond to bulk and list emails, thus preventing you from potentially spamming dozens to hundreds of people with "I'm not in my office" emails.
An example filter with bogus senders has been included in the "Example Filters" section of the filters page. In the "Matching Rules" section, you should match the Sender. For actions, you can either "Move message to" a folder of your choice or simply "Discard message." It is HIGHLY recommended that the last action of the filter be "Stop processing filters."
There are several instances where it makes sense to be 100% sure that a given filter is either the last or the only filter to act on a given mail. For example, if you had a "Blacklist Unwanted Senders" filter followed by a "Vacation Message" filter, you don't want to send your vacation message in response to an email from a blacklisted sender. By adding the "Stop Processing Filters" action to the "Blacklist Unwanted Senders" filter, we ensure that the blacklist filter is the last filter to act on a matched mail.