The Ultimate Guide to Accept-all (Catch-all) Email Addresses

An accept-all (catch-all) domain is an email domain that is configured to accept all emails, even if they’re sent to non-existent mailboxes on that domain.

Email addresses set up on such domains are called accept-all addresses. They're sometimes called catch-all addresses, and the two terms have the same meaning.

Accept-all email addresses can’t be verified with full confidence.

This means that when you send emails to these addresses, you’re more likely to get a bounce-back message. In fact, my tests show that emails sent to accept-all addresses are about 27x more likely to bounce than emails to valid addresses.

When you verify an accept-all address with Hunter’s Email Verifier, it will be classified as such so you can treat it with extra care.

(Many, but not all, other email verifiers also display this status.)

To fully understand what an accept-all address means for your outreach campaigns, you need to understand how accept-all domains work.

In this article, I’ll explain:

• What accept-all domains are,
• How Hunter verifies email accounts from accept-all domains, and
• How you should treat accept-all addresses when it comes to sending outreach campaigns.

What’s an accept-all domain?

An accept-all (also known as catch-all) domain is an email domain that is configured to accept all emails, even if they’re sent to non-existent mailboxes on that domain.

This is contrary to regular email domains that reject emails sent to non-existent mailboxes, resulting in a bounce.

This has a big impact on how email addresses can be verified. With a regular email domain, you can confirm whether a given email address exists on that domain or not by asking the server if it will accept emails sent to that address.

But since accept-all domains don’t reject any emails, you can’t check them for valid and invalid addresses on those domains.

Nowadays, the catch-all configuration isn’t always used to actually forward emails to a catch-all mailbox. Instead, its purpose is often to prevent email finders from using brute force methods to find valid addresses set up on the domain.

What happens when you send an email to an accept-all address?

Generally speaking, accept-all domains can be configured in two ways:

• They can have an accept-all mailbox like accept-all@yourdomain.com and redirect emails from non-existent addresses to that mailbox.
• They can respond to incoming emails as if they had an accept-all mailbox but actually reject these emails resulting in a bounce (I’ll later cover why that’s often the case.)

So if you send an email to an accept-all address, there are three possible scenarios:

• If that address exists, your email will be delivered as normal.
• If it doesn’t exist but the administrator sets up an accept-all mailbox, your email will be forwarded to that mailbox.
• If it doesn’t exist and no forwarding address exists, your email will bounce.

How to check if an email domain is accept-all or not

You can find out if a given domain is configured as accept-all by using SMTP tickling. This is also the method that Hunter always uses to check if the email addresses it finds or verifies are hosted on accept-all domains.

Here’s how it works:

Before an email is delivered to a recipient’s mailbox, there’s a conversation between the sending and receiving email servers.

One of the pieces of information exchanged is the recipient’s email address: the sending server specifies the recipient’s mailbox, and the receiving server needs to confirm if it is willing to accept emails sent to that address.

If a server isn’t configured to accept all emails, it will reject emails to non-existent recipients.

On the other hand, accept-all domains will respond with readiness to accept your emails regardless of whether the address actually exists or not.

Email verifiers use this exact mechanism – SMTP tickling – to determine which domains are configured as accept-all.

Hunter’s Email Verifier connects to the receiving domain and asks if it will accept emails sent to an address that doesn’t exist for sure (because it’s a random string of characters.) If the receiving server responds positively, it means that it’s configured as accept-all.

Example of accept-all vs valid email address

Let's look at a simple example to help you understand the difference between these two statuses.

Right now, my work email is ziemek@hunter.io. Hunter’s Email Verifier will report its email status as “Valid.” (See the report for yourself.)

Hunter.io is not an accept-all domain. This means that if you didn’t know my exact valid address and you tried something like  ziemk@hunter.io or z@hunter.io, your email would bounce, but if you send it to the valid ziemek@hunter.io address, it will be delivered.

However, before I joined Hunter, my professional address was ziemek@onely.com. That email domain was configured as accept-all, so Hunter appropriately displays the corresponding status for that address. (See the report for yourself.)

As far as I know, my old mailbox was redirected. So if you sent an email to that address, it wouldn't bounce, but rather get redirected to a colleague's mailbox.

However, many accept-all domains aren't configured that way and would reject your message, sending a bounce-back message.

Are emails sent to accept-all addresses more likely to bounce?

The short answer is yes – emails sent to accept-all mailboxes bounce more frequently than emails sent to valid addresses.

I ran an experiment to confirm this.

I found a couple hundred people with a similar job title in a similar industry and location.

I found their email addresses using Hunter's Email Finder.

Then, I chose 100 accept-all addresses and 100 valid addresses (control group) and I sent them the exact same email campaign.

(I used my hunter.io account so that sender reputation wouldn't be a confounding variable – this domain has an exceptionally strong sender reputation).

Then I analyzed the bounce rate for both groups.

The bounce rate was 27% for the accept-all segment, and 1% for the control group.

Needless to say, a 27% bounce rate would kill my deliverability and get my account suspended if I used a weaker domain and sent more emails.

Can you verify accept-all email addresses?

Although an accept-all address can never be verified with full confidence, there are ways to estimate how safe it is to contact:

Web sources

The first way is to check if an accept-all email address can be found anywhere on the web – on the company website, on social media, etc.

If the address pops up organically in web content, it makes it much more likely that it actually exists.

That’s one of the reasons why Hunter crawls the web looking for email addresses. If we find an address in multiple recent sources, it’s very likely that it’s safe to contact even though it’s hosted on an accept-all domain.

Based on the number and freshness of the sources we find, Hunter calculates a confidence score that can help you decide if the address is safe to contact. If the confidence score is 85-100%, you can treat the address as if it’s valid, even though it’s still slightly more likely to bounce.

Hard test

The second method involves sending an actual email to the accept-all address to see if it bounces or not.

Of course, the fact that it doesn’t bounce means it could have landed in an accept-all mailbox, which makes it unlikely that it will ever be read.

It’s also worth noting that this method raises ethical concerns as it’s putting unnecessary strain on the receiving email servers.

People chip hack

The final method of attempting to verify accept-all addresses is an open secret among outreach experts.

It involves using an obscure Google Sheets feature which converts email addresses to so-called people chips.

To use this method, you need to save an email address in a sheet cell, then right-click on the cell and select Smart chips -> Convert to people chip.

The assumption behind the method is that if the accept-all address is valid, it will be successfully converted to a people chip.

I tested the people chip method to see if it’s really effective in verifying accept-all addresses.

The conclusions are:

• For Google-hosted email addresses, the people chip method is very effective. In my experiment, I got a 0% bounce rate when sending emails to addresses that I successfully converted to people chips. On the other hand, the non-converted email addresses bounced back 56% of the time.
• For email addresses hosted by ESPs other than Google, the people chip method isn’t as effective, but still worth your consideration.  In my experiment, 4% of my emails bounced when sent to non-Google addresses verified with people chips. On the other hand sending emails to addresses that I successfully converted to people chips. On the other hand, I got a 32% bounce rate with non-Google addresses that didn’t convert to people chips.

How to send outreach campaigns to accept-all email addresses?

You must be mindful when using accept-all email addresses in your cold emails.

My tests show that they’re much more likely to reject your emails, which can be terrible for your sender reputation.

This means that sending emails to accept-all addresses without any precautions will also damage your deliverability with valid recipients!

On the other hand, with accept-all addresses being a significant chunk of all addresses you’ll find, you can’t just toss them – you’d be foregoing a big portion of your total addressable market.

Here are some tactics you can consider to still contact these recipients:

1. Review Hunter’s confidence score. If it’s between 85-100%, you can use it in your campaigns, but make sure you monitor your bounce rate and keep it under 2%.
2. Verify Google-hosted addresses using the people chip method. If they are successfully converted to people chips, you can safely contact them.

I created a spreadsheet you can use to automatically check the MX records of your accept-all addresses and quickly filter the Google-hosted addresses that are successfully converted to people chips. I also created a video walkthrough which you can check out on LinkedIn.

If Hunter’s confidence score is low and you can’t rely on the people chip method, you still have two options:

1. Add the accept-all addresses to your mailing list, but only make them 2-5% of the list to account for the higher risk of bouncing.
2. Consider different ways of contacting these prospects (e.g., LinkedIn.)

Wrapping up

Accept-all (catch-all) domains are tricky to understand.

On the one hand, they are configured to say yes to receiving emails even if they’re addressed to non-existent recipients.

But on the other hand, they are more likely to reject your emails and grow your bounce rate.

Since accept-all mailboxes are bound to constitute a significant portion of your total addressable market, you need a strategy for contacting them while protecting your email deliverability.