Official comment
We check to see that the email address is properly formed far beyond that it has a @ symbol. There are a wide variety of characters that can be used in a properly formed email address, such as special characters !#$%&'*+-/=?^_`{|}~ and some UTF-8 characters.
We are implementing additional "list bombing" checks against email addresses to see if they are from known spammers, IP addresses used by spammers, or are "too common" (too many submissions). These are valid email addresses that are suspect.
Refer to our article on list bombing at https://support.ontraport.com/hc/en-us/articles/115001551294 for information on how to identify the email addresses. We don't delete your data, but provide a way for you to identify the suspect email addresses and make the decision yourself.