Mail aliases are additional mail addresses that you can use with the same mailbox without having to switch accounts. Aliases are a premium feature. If you upgrade to Premium (€1 per month), you can add up to 5 aliases.
Please login via the new mail client to add aliases as the process in the new client is much easier. Go to 'Settings' → 'Email' → 'Email aliases': Click on 'Show Email Aliases'. Click on the plus symbol to add aliases. A pop-up opens where you can type the alias you want to add. Click on the three-dot button to choose the domain for your alias. This can be any of the Tutanota domains or of your own domains that you have added to your Tutanota account.
Please note: Aliases with a Tutanota domain can only be disabled, but not removed. Deactivated aliases must remain linked to your account in case you want to activate them again in the future. When you are using your own domain with Tutanota, you can delete aliases with your own domain and create new ones.
You can change the default sending address to your own domain alias (or any other alias) by changing the default sender in 'Settings' → 'Email' → 'Default Sender'. This will make your alias the default sender. However, the main address of your Tutanota account (name in tab) will remain unchanged.