Hey @Caroline Gomez 

There are a few troubleshooting steps you can take here:

• First, ensure that the custom sender address has been verified. This is a necessary step before sending emails from that address.
• Double-check that you have followed all the steps outlined in the help documentation for verifying your domain. It's important to verify that the DNS records are set up correctly, especially the DKIM records. Some DNS providers require only the subdomain part of the record, so make sure to check if you've set it up with the shorter or longer version, and that it matches your domain provider's requirements.

By following these steps, you should be able to resolve the issue with sending emails to new users. However if the problem persists after these checks, it may require further investigation by our support team You can open up a new conversation and reference this ticket so that your question is handled by the appropriate team.

Let me know if the above was helpful!