This seems to be a really annoying issue, especially for those who use custom emails for their Twitter accounts. Twitter is now suspending accounts with custom domains as spam, especially those created after 2020. This issue could also be due to security checks, unusual activity, or policy violations. It is also important to know if all accounts have received the same message. It would be best to check Twitter’s email and file an appeal with official support. Hopefully, this is an automated system error and the accounts will be restored soon.