I get “SMTP connect() failed” or “535 Authentication failed”, how do you fix it?

Those errors mean Magento can reach (or can’t reach) your SMTP relay but the login is being refused. Common causes: wrong port (use 587 with STARTTLS or 465 with SSL), an account password used where the provider requires an app password or API key (Gmail/Office 365), the host blocking outbound port 25/587, or a missing SMTP extension. We configure the SMTP module for your relay (SendGrid, Mailgun, Amazon SES, Postmark, Office 365), fix the TLS handshake, and send a test until we get a clean 250 OK.