Added some helpful comments to the xml and JS.

bemade_mailcow_blacklist/static/src/js/mailcow_mailbox.js

@@ -5,16 +5,19 @@ import ListView from 'web.ListView';
 const viewRegistry = require('web.view_registry');
 
 const MailboxesListController = ListController.extend({
+    // buttons_template must match the t-name on the template for the button (static xml)
     buttons_template: 'mail.mailcow_mailbox_list_view_buttons',
     events: _.extend({}, ListController.prototype.events, {
         'click .o_button_sync_mailboxes': '_onSyncMailboxes',
     }),
+    // This may need to be async function() if there needs to be an await this._rpc({ ... }); call to not reload early
     _onSyncMailboxes: function () {
         this._rpc({
             model: 'mail.mailcow.mailbox',
             method: 'sync_mailboxes',
             args: [],
         });
+        // Couldn't test this for real, but it runs the action. May need a this.reload()
     },
 });
 
@@ -24,4 +27,5 @@ const MailboxesListView = ListView.extend({
     }),
 });
 
+// key must match with the js_class attribute of the tree view you want to modify
 viewRegistry.add('mail_mailcow_mailbox_tree', MailboxesListView);

bemade_mailcow_blacklist/static/src/xml/mailcow_mailbox.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <templates>
-    <t t-extend="ListView.buttons" t-name="mail.mailcow_mailbox_list_view_buttons">
+    <t t-extend="ListView.buttons" t-name="mail.mailcow_mailbox_list_view_buttons"> <!-- t-name must match with js controller button template -->
         <t t-jquery="button.o_list_button_add" t-operation="after">
             <button type="button"
                     class="btn btn-primary ml4 o_button_sync_mailboxes"

bemade_mailcow_blacklist/views/mailcow_mailbox_views.xml

@@ -4,7 +4,7 @@
         <field name="name">mailcow.mailbox.view.tree</field>
         <field name="model">mail.mailcow.mailbox</field>
         <field name="arch" type="xml">
-            <tree string="Mailbox" decoration-info="active == False" js_class="mail_mailcow_mailbox_tree">
+            <tree string="Mailbox" decoration-info="active == False" js_class="mail_mailcow_mailbox_tree"> <!-- js_class must match the name added to the viewRegistry in js -->
                 <field name="active"/>
                 <field name="name"/>
                 <field name="local_part"/>