Custom Bounce Domains and SPF Authentication

The Custom Bounce Domain is how SPF is implemented with SocketLabs. This is done without needing to adjust your existing SPF record.

Bounce Addresses

Email messages contain two "from" addresses, the Return Path and the Friendly From address.

 The return-path address is also known as the Mail From, Envelope From, 5321.from, or the Bounce address. This address is not shown to the recipient and is the designated email address where bounced messages and other feedback is sent. In this article, we'll refer to it as the return-path.

The Friendly From address is also known as the Display From or 5322.from. This is the address that is shown to a recipient.

The return-path address is what a receiving server checks to verify the SPF record. The receiving server does not check the friendly-from domain, so an SPF record on the friendly-from is not necessary.

Custom Bounce Domain Setup

1. Log into your DNS provider for your sending domain.

2. Create a subdomain. 

3. Point a CNAME for that subdomain to

To set up the custom bounce domain for, the following CNAME record would need to be configured: CNAME

This record tells DNS systems that when looking up, they should redirect and look at the records for

Modifications to DNS settings can take up to 48 hours to propagate, but generally will propagate within minutes.

Return-Path and Custom Bounce Domain: Examples

Sending without Custom Bounce Domain set up in SocketLabs

If you do not set up any custom bounce domains, we will use as the default value. Each server may only have a single default value configured.

This is a message that does not have any custom bounce domains set up:

Return-Path: <>
From: Your Company <mycompany@something.example> via

In this example, the receiving server will look at the return-path domain and will look for the SPF record belonging to 

If you do not set up your custom bounce domain, your recipients are likely to see an additional "via" in the sender details of an email.

Sending with Custom Bounce Domain set up in SocketLabs

If you have set up your custom bounce domain (in this case, bounces.something.example) to point to, you will no longer see "via" and will have successfully set up SPF authentication:

Return-Path: <8c229047f6aa6add74c68806d0589331@bounces.something.example>
From: Your Company <mycompany@something.example>

In this example, the receiving server will look at the return-path domain, see that bounces.something.example points to, and will successfully authenticate.

Multiple Custom Bounce Domains

You can add multiple custom bounce domain entries in the SocketLabs Control Panel. We will automatically match your from address to the appropriate custom bounce domain entry. If no match is found, we will use either your selected default custom bounce entry or we will use as the default.