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Exchange Online Protection is currently broken. Resolved.

Intro
6/24th, 3:20 PM
A lot of my outbound emails are currently stuck with this status:

Deferred: 421 4.3.2 The maximum number of concurrent server connections has exceeded a limit, closing transmission channel

For instance, to belcorp.biz.

The MX record:

> dig +short mx belcorp.biz
10 belcorp-biz.mail.eo.outlook.com.

So it ends in outlook.com.

Same for emails to accenture.com. The MX record:

> dig +short mx accenture.com
10 accenture-com.mail.protection.outlook.com.

Also ends in outlook.com.

sheraton.com is another one I see.

So all these domains we can’t currently email to have an MX record ending in outlook.com and so I conclude are using Exchange Online Protection.

The situation has persisted for a couple hours so this doesn’t seem to be a 99.9999% uptime type of service.

Conclusion
Something went seriously wrong with Microsoft’s Exchange Online Protection service today.

6/25/14 update
Apparently this affected lots of Outlook users as well. It was finally fixed last night.

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