SMTP Error 550 5.1.1: Recipient address rejected: user unknown

Identify: does your log match?

550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. For more information, go to
550 5.1.1  https://support.google.com/mail/?p=NoSuchUser 41be03b00d2f7-7ea9c6a1b2csi98765432a12.401 - gsmtp

1. High

   Typo in the address

   gmial.com, missing letters or a trailing space at signup. The domain may exist but the local part does not.

2. Medium

   Deleted or deactivated account

   Employees leave and corporate mailboxes are removed; old lists contain many such addresses.

3. Low

   Catch-all disabled after migration

   A domain moved providers and only some aliases were recreated.

Fix

for (const user of users) {
  await sendEmail(user.email, digest);
}

for (const user of users) {
  if (await db.suppression.findUnique({ where: { email: user.email } })) continue;
  try {
    await sendEmail(user.email, digest);
  } catch (err) {
    if (err.responseCode === 550 && /5\.1\.1/.test(err.response)) {
      await db.suppression.create({ data: { email: user.email, reason: 'hard_bounce' } });
    } else throw err;
  }
}

Verify: reproduce it on purpose

$ swaks --to this-user-does-not-exist-8f3a91@gmail.com --server gmail-smtp-in.l.google.com --port 25 --quit-after RCPT

Prevent

• Use double opt-in for signups.
• Process ESP bounce webhooks and suppress on hard bounce.
• Keep hard-bounce rate under 2%.