My PHP-based app suddenly throws “Expected response code 235 but got 535 – Authentication credentials invalid” every time it tries to send mail. I already updated the SMTP username and password, yet both LOGIN and PLAIN still fail. What I need from you is a rapid deep dive into the SSL/TLS handshake, the SMTP dialogue, and any server settings that might be blocking correct authentication. I’ll grant temporary access to the mail-sending config and stream live logs; I can also run telnet or openssl commands on demand so you can trace the exact exchange. Deliverables • Identify the precise cause of the 535 failure • Implement or walk me through the fix until a test message returns code 235 (success) • Provide the confirmed working host, port, encryption type, and auth method so I can mirror them in other environments Acceptance Criteria • A verified test email delivers successfully from the application, showing a 235 response in the log • All new settings are documented clearly enough for me to reproduce them elsewhere If you’re comfortable troubleshooting SMTP over SSL/TLS, parsing verbose openssl output, and tweaking PHP mail libraries, let’s get this gateway accepting mail again today.