Hello saorizmo,
You'd like to know how you can separate your custom domain email with iCloud. We'll do our best to assist with this.
Do you use any rules to filter the iCloud emails? If not, we'd suggest doing this to make it easier or more organized. Learn how here: Set up rules to filter email in Mail on iCloud.com - Apple Support
Set up rules to filter email in Mail on iCloud.com
You can set up rules to filter incoming messages or reorganize messages you’ve already received. For example, you can create a rule that sorts messages from a particular sender into a specific folder. You can edit and delete these rules at any time.
Set up rules to filter email
1. In Mail on iCloud.com, click at the top of the Mailboxes list, then choose Preferences.
2. Click Rules, then click “Add rule.”
3. Below “If a message,” use the pop-up menu and text field to specify the type of messages you want to filter.
4. Below Then, use the pop-up menus or fields to specify an action for the rule.
5. Click Add.
You can have up to 500 rules.
Important: If you apply rules to a folder and then delete the folder or change its name, make sure to update the rules for those folders accordingly. For example, you can’t forward messages to a deleted folder.
Would that suffice? Let us know.
Cheers!