Nodemailer versions up to 10.0.1 improperly handle deeply nested recipient arrays in fields like to, cc, and bcc. This flaw causes a stack exhaustion denial-of-service (DoS) when processing such inputs, leading to a RangeError and potential termination of the Node.js process. The vulnerability arises because only the outermost array is flattened before passing nested arrays to a parser that converts them to strings recursively, exhausting the V8 call stack. No SMTP server or successful email delivery is required to trigger this. The issue is fixed in version 10.0.2.