Sad news: Contabo throttling SMTP connections on VPSes

Two days ago we migrated a customer’s Zimbra mailserver to a SSD VPS of our beloved Contabo hosting. While I wasn’t in charge of the migration I couldn’t refrain to tail zimbra.log to see how/when mail was flowing throug, and I immediately spotted some Connection refused errors for different hosts, even our own mailserver still on a Contabo VPS.
While quite worring I let the balls stop before raising an alert.

The day after the migration there were still Connection refused errors in mail log: time to investigate.

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