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Server-side filtering of incoming email

You can sort, arrange and filter incoming email in your client however you like, but then you can only operate on messages already delivered to your Inbox. Even if your client supports automatic rule-based filters, you depend on having this particular client up and running (or at least launched from time to time). This might work well if your workflow is tied to a particular computing device, but becomes inconvenient if you are used to accessing your email from more than one place.

Fortunately, we are able to offer you a better way: you can set up rules on the mail server to sort and filter messages right as they come in to your account. Your rules are evaluated as part of incoming message delivery, so each message will arrive in the correct (sub)folder right away. Rules are specified in a language called Sieve (RFC), and can be accessed via a protocol called ManageSieve (RFC).

To access and edit your filters, you need a client that speaks the ManageSieve protocol, which allows you to upload/download scripts to and from the mail server, and designate one of the uploaded scripts as the active script to be used. You also need a way to come up with the actual Sieve script contents, which is ultimately plain text (Sieve is a simple, domain-specific programming language easy to write by hand, but there are graphical editors as well). Some examples of Sieve scripts might help to get you started.

This is a technology stack based on open standards, so you are not bound to use any specific implementation. The following hints might be helpful to get started. Please note that we are not endorsing any of the below programs, merely listing them for your information. (We did test them to verify that they work with our service, but software moves quickly, things can break, etc.) This is just our sample; there are further clients with support for Sieve.

• More accomplished mail clients usually include the ManageSieve part. For example, Thunderbird and Claws-mail both offer setting up a ManageSieve connection. Other, more minimalistic email clients do not offer any support.
• Some of these clients offer only basic support: a text-based editor of the Sieve source. You are expected to be comfortable writing your own Sieve scripts. Claws-mail is one such example via its ManageSieve plugin.
• Other clients offer a graphical editor to aid the user in assembling the ruleset that will be codified into a Sieve script. Direct editing of the Sieve source is also supported. One example is the Sieve add-on for Thunderbird. To install it, open the extension manager within Thunderbird and search for an add-on called Sieve.
• The above add-on is also available as a standalone application (Electron-based). This might come handy if you do not intend to use Thunderbird just for the sake of managing your filters.
• If you are looking for a standalone, command-line ManageSieve client (not tied to any mail client), check out sieve-connect. It is a simple, bare-bones client to access your Sieve scripts on the server – you are expected to write the scripts with your favourite text editor.

Any scripts you set up will be run in between some other site-wide scripts that you cannot change, and will be evaluated for each incoming message that reaches them. Mail accepted by the MX but classified as spam is directed to the Spam folder as part of site-wide scripts that get to run before your own rules are evaluated, which has the (intentional) implication that your filters won't see mail classified as spam. On the other hand, your rules run before the site-wide script for delivering subaddressed email, so you can override or augment that behaviour.