But this time I know why, we've exceeded the email capacity of the serving that I bought - but that's a success! I'll go and buy some more email.
OK, it turns out that I need a completely different mail service. This one only allows 500 messages a day, which we just exceeded.
It turns out that the solution is to run an email server on this host, in the meantime I'll just turn off email, which will require a reboot at 1515 UTS