Finally resolved for this client a few minutes ago, and CSR explained that the problem may have been caused by a sudden change from port authentication to username authentication and may be affecting many customers.
We were able to patch up MX records to keep mail flowing, and the backup MX service at Freeparking was once again proved invaluable.
If this issue affected many customers from yesterday then you would think there would be more noise level? Maybe the problem commenced earlier than that, and is only apparent with random reconnects of the service?
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