Our website(shopify website) has been getting a bot attack on our newsletter subscription form. Not sure if emails are real or not but there is some from gmail and hotmail etc or some from weird domains such as frettin.is or libero.it. Most emails have "+" sign random numbers. For example "examplebot+34343@weirddomain". The names and surnames are just random letters and numbers.
When we checked CleanTalk with these emails, they were added to blacklists very recently.(13-14 nov, attack happened on the same dates)
We have since removed the newsletter form and added captcha to rest of the forms in the site. No new bots have signed since then
Main problem here is that when we check these accounts in our CRM(Hubspot) it says they were created from "offline sources". Similarly there are no ips or any weird country activity on Google Analytics.
What could be the source of this attack, cause and the reason?