I just found that "No Email - Of Any Kind" was enabled after discovering that even the normal option to send an invoice was not there. I don't recall ever setting "No Email - Of Any Kind". As it turns out, an email to one of the customer contacts bounced which then activated this setting.

The problem is that one could be updating tickets and not even realise that the emails are not actually being sent, because the ticket comment box still shows the button with "Email" on it along with the customer's email address below the button.

It would be extremely useful if when the "No Email - Of Any Kind" is enabled the ticket comments box indicated it. One indication could be to replace the orange email button in the comments box with "No Email - Of Any Kind ENABLED" in RED.