*Update*

ITServices found an application issue and has restored external email delivery. Messages that were sent during the outage should be queued by the sending server and delivered at a later time.
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Previous Description (2011-02-28 14:00:12):

There is currently an issue with off campus email delivery to departmental email servers such as appsci.queensu.ca, business.queensu.ca and others that route through extmail.queensu.ca mail filter.

ITServices is currently investigating the issue.