*Update*

ITServices has confirmed with the provider that Queen's email service is no longer on the list. Anyone previously affected by this should no longer have issues sending to external recipients.
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Previous Description (2014-04-17 11:53:51):

This notification is to let you know that, due to some compromised email accounts, Queen’s has been “blacklisted” by a black list provider. This means that Organizations who subscribe to this provider will automatically reject email messages sent from Queen’s accounts.

If you receive an email that looks like the one below, it means that your email message has been rejected. You do not need to do anything, but you will need to remember to resend your email once Queen’s has been taken off the providers blacklist. ITServices is currently working with the provider on this issue, and we will let you know when it has been resolved.

Sample rejection email:

This is the Spam & Virus Firewall at bc600c.its.queensu.ca.

I'm sorry to inform you that the message below could not be delivered.
When delivery was attempted, the following error was returned.

example@example.com>: host mail.example.com[555.555.555.555] said: 554
5.7.1 Service unavailable:client host[??]blocked. (in reply to RCPT TO command)

Thank you for your patience.