Permission mode controls which of your destinations are allowed to reply or send emails through an alias. It's a security layer — without it, anyone who knew your alias address could potentially spoof it as the sender.
The two modes
Open mode — any of your verified destinations can reply or send from this alias, as long as the reply or send feature is enabled on that destination. Less restrictive, more flexible.
Strict mode — only the specific destination the alias is assigned to can use it for replies or sending. Any other destination's attempt to use the alias is blocked.
Which to use
For most people, strict mode is the right default. Your aliases are typically tied to one destination inbox anyway, and strict mode makes sure only that inbox can act as the alias.
Open mode makes sense if you have multiple destinations and regularly want to reply from an alias no matter which inbox you're reading from.
Setting the mode
Account-wide default — Go to Settings → Aliases and find Alias Permission Mode. Set it to strict or open. This applies to all aliases that don't have an override.
Per-alias override — On any alias detail page, there's a permission mode setting in the Settings section. You can override the global default for that specific alias.
This only applies to replies and sending
Permission mode doesn't affect whether emails are received through an alias. Inbound forwarding always works as long as the alias is active. Permission mode only controls outbound — whether you can reply to or initiate emails from the alias address.
To enable replying from a destination, see Destinations and toggle on Can Reply for the relevant destination.