421 4.7.66: TLS 1.0 and 1.1 are not supported

Microsoft 365SMTPSign-in problem

The error usually looks like this:

421 4.7.66 TLS 1.0 and 1.1 are not supported. Please upgrade/update your client to support TLS 1.2.

What it means

Your app is using an outdated version of TLS encryption that Microsoft 365 no longer accepts.

Common causes

  • The app, device or operating system only supports TLS 1.0 or 1.1 (common on older printers, scanners and Windows versions).

How to fix it

  1. Update the device firmware or app, or enable TLS 1.2 in the operating system.
  2. If the device cannot be updated, Auth-Email’s legacy servers accept older TLS versions from your device and connect to Microsoft 365 using modern TLS and OAuth.

See also our guide: Legacy server settings.

Tools to help diagnose it

SMTP connection tester

Spin up a real SMTP test server and diagnose your email sending configuration in real time

Email server checker

Check any IMAP, POP or SMTP server’s encryption, certificate and supported sign-in methods

Can’t update your app to use OAuth?

Auth-Email fixes sign-in errors like this one without changing your app, printer, scanner or script. It signs in to Microsoft 365 with OAuth on your app’s behalf, while your app keeps using a normal username and password over IMAP, POP or SMTP.

Read the docsTry Auth-Email →

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