Hi, @micapiacenza. It look like you are running into issues with the previous DNS records sometimes being returned due to time to live (TTL) issues as mentioned above. I don’t think the 403s are from Netlify but are instead coming from some previously used service.
About getting the email to work again, there is a support guide about this here:
There are three solutions in that section above. I would recommend solution 3 in this situation. Revert back to your registrar’s DNS service and then use the external DNS instructions instead:
If there are any questions about that solution or the other two (whichever you are most interested in), please let us know.