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.