This might be helpful for those experiencing similar problems. I received it from MailChimp support:
Thanks for reaching out to us at MailChimp Support. It is concerning to hear that subscribers are not receiving confirmation emails, however I would like to provide assistance.
I went ahead and signed up to your list using an example email address, and also was not able to retrieve any confirmation emails. However I was able to confirm that our system is logging the emails as sent, so the receiving email servers are initially accepting them.
I then checked the “from” email address set for your lists, and see that it is set to [email protected]. Delivery issues can happen when you are using a free email provider (such as Yahoo.com, Gmail.com, etc.), especially a Yahoo email address.
Authentication and Yahoo DMARC Policy
To clarify, DKIM and SPF authentication is used by receiving email servers to catch email spam and spoof email addresses, and includes verifying the location the email is sent from. While the “from” email address says “[email protected]”, the receiving server is able to tell that the email is not actually being sent from Yahoo.com, but from our email servers and domain. Whenever an email claims to be “from” a domain, but not sent from that same domain, it is a valid concern for receiving email servers.
How Free Email Providers Affect Delivery: https://eepurl.com/b43LoD
About Email Authentication: https://eepurl.com/b43Lpb
This is especially relevant as free Yahoo accounts on the Yahoo.com domain have strict rules when trying to use an email proxy service to send on behalf of an email that belongs to its domain (it is not allowed). And while we do change your email address so receiving servers do not consider the “from” address to be “Yahoo.com”, the receiving server still may be able to detect the Yahoo.com portion of the email and and reject it.
Yahoo DMARC policy: https://help.yahoo.com/kb/SLN24050.html
Changing the “From” email address for your list
You may be able to switch your account email to another free email provider (a Gmail “from” address may work better) to improve delivery.
To change your “from” email address:
Select “Lists” at the top of your account, then select your list
Select the “Settings” drop-down tab, then select “List name and defaults”
Change the “from” email address in the “Default From email address” field
How to Edit Your From Email Address: https://eepurl.com/cPUXuf
Verify a domain for sending with MailChimp
While you can use another free email service, I would specifically recommend using an email address that has been setup on a custom domain, then verifying it with us and setting up Authentication for your domain.
Select your profile drop-down button in the top right hand corner of the screen, then “Account”
Select the “Settings” drop-down tab, then “Verified domains”
Select the “Verify A Domain” button, enter the email address for your domain, then “Send Verification Email”
Open up the email account for your domain, and look for the verification email from “[email protected]”.
Open the email and select the “Verify” button to finalize the verification of your domain (if needed, you can copy the code provided and use it in your account to verify the domain)
Verify A Domain: https://eepurl.com/b43LqP
Authenticate a domain for sending with MailChimp
Select your profile drop-down button in the top right hand corner of the screen, then “Account”
Select the “Settings” drop-down tab, then “Verified domains”
Select “View setup instructions” on the “Authentication” step
Follow the provided steps to add a CNAME record to your domain, and TXT record to your domain, granting DKIM & SPF authentication for MailChimp
Select “Authenticate Domain”
Set Up Custom Domain Authentication: DKIM and SPF: https://eepurl.com/b43Lqb
I hope this helps to clear things up with the deliverability of your confirmation emails, and helps better explain how delivery generally works when using a bulk-sending email service such as ours. Feel free to let me know if you run into any other questions, otherwise I hope everything else is well for you and wish you all the best.