When attempting to activate an inactive user in FlexNet Operations the following error is produced: “Error has been returned by the Mail Service : No MimeMessage”.
This type of error typically occurs if the email template content is empty. In this case, the "User Enabled" email template has no content and send indicator is true.
Add the required content to the "User Enabled" email template body or set the "send indicator" to false.
This error can occur with other email templates too. Above solution can be applied as required.