If you're getting an error like "Authentication failed", the server can't recognize you. This is usually a simple settings or password issue.

1. Username: Not just the name, but your full email address. This is the most common mistake.
2. Password: Try signing in at https://mail.gelecekint.com. If you can get in, the password is correct and the problem is in your program's settings.
3. Server: Both incoming and outgoing must be mail.gelecekint.com.
4. Port/encryption: Incoming 143, outgoing 587, with encryption off on both. If SSL/TLS is selected by mistake, it will fail.
5. SMTP authentication: "Server requires authentication" must be checked, using the same username/password as incoming.

If you just changed your password, update all your devices; when copy-pasting, make sure no space slipped in and check that Caps Lock is off. Too many failed attempts can temporarily block your IP. If it persists, write to destek@gelecekint.com with the program name and a screenshot of the error.