Improper Check or Handling of Exceptional Conditions in nodemailer

Summary

In nodemailer versions through 7.0.10, a Denial-of-Service (DoS) can occur that immediately halts the system due to the use of an unsafe function. According to RFC 5322, nested group structures (a group inside another group) are not allowed. Therefore, in "lib/addressparser/index.js", the email address parser flattens nested groups when they appear, since such input is likely abnormal. (If the address is valid, it is added as-is.) In other words, the parser flattens all nested groups and inserts them into the final group list.