Snappymail is an excellent webmail. To enable sieve on snappymail, you should install dovecot-sieve at first.

 sudo apt install dovecot-sieve dovecot-managesieved

Then edit file "/etc/dovecot/conf.d/20-lmtp.conf" and update this section:

 protocol lmtp {
   postmaster_address = [email protected]
   mail_plugins = $mail_plugins sieve
 }

And edit file "20-managesieve.conf" to update this section:

 service managesieve-login {
   inet_listener sieve {
     address = 127.0.0.1
     port = 4190
   }
 }

This setup managesieve to listen on localhost only.

In my practice no more other changes were needed.

Then restart dovecot.

Open the management panel for snappymail, in domains section, pick up the domain you want to update.

For that domain, enable sieve as follows.

Now log into snappymail webmail, in its settings, you will find "filters" option.

Click "add a script" to add a sieve filter, like the following.

The above script says, if the recepient domain is pobox.be, then deliver it to pobox folder. If the recepient domain is simplemail.co.in, then deliver it to simplemail folder.

You must know something about the sieve script.

After you have added a script, please make sure that script is in active status (it should be selected).

Reference:

Sieve Tutorial

Sieve Mail Filtering Setup