The most common error when setting up email sending via Google SMTP is caused by Google's security settings.

java.rmi.ConnectIOException: I/O error ocurred during RMI call; nested exception is: java.rmi.RemoteException: Exception thrown by remote method 'sendMail'; nested exception is: cz.winstrom.service.mail.MailException: javax.mail.AuthenticationFailedException

To use Google's SMTP server, you need to change the settings in the Gmail client application for the relevant email account. Navigate in the menu to GSuite -> Manage Google Account -> Security -> Less secure app access

Note: This setting cannot be enabled when two-factor authentication is active at the same time.

Read more information about SMTP configuration in Flexi.