Go to Settings → Aliases. These are the defaults that apply to every new alias you create. You can always override them on individual aliases.
Default permission mode
Sets whether new aliases start in Strict or Open mode.
Strict — only the destination the alias is assigned to can reply or send through it.
Open — any destination with the right permissions can use the alias for replies and sending.
Pick whichever matches how you work most of the time. You can change the mode on any individual alias regardless of this default. See Alias permission mode for a full explanation.
Default banner setting
Controls whether the forwarding banner is shown by default on emails forwarded through new aliases. The banner is a small header that appears in forwarded emails showing which alias the message came through.
You can override this per-alias. See Per-alias banner settings.
Require explicit recipients
When enabled, an alias only forwards email from senders you've explicitly approved. Email from senders not on the list is blocked. This is a stricter privacy mode — useful if you want very tight control over what reaches your inbox.
This can also be toggled per-alias.