The account belonged to Synchronoss Technologies, which took over the running of BT’s cloud services last month. While BT did not provide details on the reason for the disruption, it appears to be something gone wrong during testing.
It appears that outgoing messages were getting forwarded to email@example.com address and that was bouncing email as the mailbox was full. According to Linked in there is a Steve Webb who supports O2 and BT email platforms looking after the mail gateway, the backend servers containing the mailboxes, calendars and address books and other servers in the platform.
“I have supported 28 million email accounts and 80 million emails per day,” if we are right then this guy literally did the job for a few hours this week.