Sending email with SMTP and Deno, with InReplyTo headers failing

import { SMTPClient as DenoMailerSMTPClient } from "https://deno.land/x/denomailer@1.6.0/mod.ts";

type Smtp = {
  host: string;
  port: number;
};

type EmailObject = {
  from: string;
  cc?: string;
  subject: string;
  content: string;
  inReplyTo?: string; // Added inReplyTo field
  references?: string; // Added references field
};

export async function send_smtp_email(
  smtp_res: Smtp,
  to_email: string,
  from_email: string,
  subject: string,
  content: string,
  cc_email?: string,
  inReplyTo?: string,
  references?: string,
) {
  const client = new DenoMailerSMTPClient({
    debug: {
      allowUnsecure: true,
    },
    connection: {
      hostname: smtp_res.host,
      port: smtp_res.port,
    },
  });

  const headers: { [key: string]: string } = {
    from: from_email,
    to: to_email,
    cc: cc_email || '', // Use the cc_email parameter here
    subject: subject,
    inReplyTo: inReplyTo || '', // Add In-Reply-To header if provided
    references: references || '', // Add References header if provided
  };

  await client.send({
    headers,
    content,
  });

  await client.close();
  return `Email sent from ${from_email} to ${to_email}`;
}

export async function main(email: EmailObject, smtpResource: Smtp) {
  if (email != null) {
    const currentDate = new Date().toLocaleDateString("fi-FI");

    const emailBody = "Hei"
      `On ${currentDate}, ${email.from} wrote:\n` +
      `\"${email.content}\"\n\n`

    const sendEmail = await send_smtp_email(
      smtpResource,
      email.from,
      "sender",
      `Re: ${email.subject}`,
      emailBody,
      "cc",
      email.inReplyTo,
      email.references,
    );

    return sendEmail;
  } else {
    return "NOOP";
  }
}