1. A “Server says: 535 – Incorrect authentication data” error is usually related to the SMTP User Authentication details you have configured (or not configured) in your email settings.
2. This error is often reported by mail users who maintain multiple mail devices, and forget to update login details in one of their devices.
3. However, in very rare cases 535 Incorrect Authentication Data’ results when mailbox permissions are changed due to migrations, or an account/server update etc. I have ran a "Repair Mailbox permissions" which should resolve the issue if that's the case.