Hey @jamieedmondson, Optus email accounts, do come at a capacity of 100MB. Once you reach this limit, all incoming emails will be halted till more space is made.
My best recommendation from here, would be to clear any unwanted emails, including those in your spam folders and your email functions should restore soon after. Give those a shot and see how you go -Quade
Did we answer your question? Please mark it as a Accepted Solution and if you see something that was helpful to you or the rest of the community, giving a Kudos is only a click away.