I seem to be getting this every 2-3 days. It works for a while, then after sending 10-20 emails, the rest get blocked! It's usually resolved the next day, but I am stuck until it's resolved.
Am using the Outlook client on a laptop. Have tried contacting Optus and they said it's a problem with my Outlook, but how can that be when it was working properly for a couple of days, then stops! It's been so frustrating!