Welcome to the Community @klokanik
Before attempting the troubleshooting steps, TPG recommends that you:
Once confirmed that your service is not affected by an outage, you may proceed with the troubleshooting given in this article : Cannot Send Email Rejected by the Server/Banned from using SMTP,
Also, make sure that your email settings are correct. Check this link for reference.
Feel free to search Community for help with your issue, or contact us and we'd be glad to assist you.