Problem: You want to use the most popular free email provider in China with OX and already set up the mail account in OX->Settings->Mail->Account, but you are not able to send emails from Open-Xchange WEB GUI and get an error when sending emails in the GUI, like "553 authentication is required,smtp4,jdKowLDrf6dIp5FNlq1mBQ--.1474S2 1301391179"
Solution: OX normally tries to connect via SMTP port 25 and automatically switches to START TLS to make the connection secure. The 126.com smtp server seems to be broken when using TLS. To fix that, go to OX->Settings->Mail->Account and open your 126.com account settings. Now check that you have enabled "Use SSL connection" for sending emails via SMTP and that the port numer is set to 465. Also make sure that the login is correct and you have inserted the complete 126.com email address.