(This would happen if you were using Internet Explorer to send messages. If you switched browsers to Google Chrome or Mozilla's Firefox you could send messages normally.)
What gives? There was some legacy code that determines which browser you're using to see if your browser supports a more advanced text editor for typing email messages. If so, you get the more advanced editor. Overtime, browsers have changed, but the code remains the same, and here lies the issue(s).
I'm happy to report that the offending code has been updated and you can send messages from Internet Explorer again.
Here's how you send messages from a course or group in MyGateway
Click Email link.
Choose the recipients, and click the "Send E-mail" button.
Compose your message and click the "Send" button to send to all recipients.
**Note: the final step is where the process would fail.
