# Load balancing
Postal route load balancing has three primary use cases:
{% hint style="info" %}
**Contact List Domain Grouping**
Route mailbox providers that have multiple domain names sharing a common email infrastructure through an API, SMTP Relay, or MTA delivery policy optimized for them using [contact list domain routing](https://docs.emaildelivery.com/docs/advanced-postal-route-configuration/contact-list-domain-routing).
{% endhint %}
{% hint style="info" %}
**Optimizing for deliverability**
Route important mailbox providers like Gmail through the API, SMTP Relay, or MTA delivery policy where you're getting the best deliverability**\*.**
{% endhint %}
{% hint style="info" %}
**Partitioning**
Distribute your outgoing mail across multiple API, SMTP Relay, or MTA delivery policies based on percentage allocation and warmup status.
{% endhint %}
{% hint style="success" %}
\*Delivery is when a message is successfully delivered, Deliverability is when the message hits the inbox.
{% endhint %}
{% hint style="warning" %}
**One sender email address using multiple email service providers**
The email address configured in your broadcast or funnel will be used when load balancing multiple API or SMTP Relay connections.
This email address will likely need to be authenticated with each email service provider first.
{% endhint %}
{% hint style="info" %}
Contact list domain routing will be disabled by default with the default setting of "All Domains in Contact List" sending all mail through one route.
{% endhint %}
To route your mail based on the mailbox provider's domain, click Add Domain Route.
Use the dropdown to select a [domain group](https://docs.emaildelivery.com/docs/advanced-postal-route-configuration/contact-list-domain-routing) you've created previously.
Select an alternative path for delivering mail to this domain.
Repeat until your postal route is fully optimized according to your delivery strategy.
Use **partitioning** to split your email for the *same* domain group across multiple email service providers or MTA delivery policies.
