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Yes, you create an Atlassian account using a non-NUS email address. In fact, this is preferred. Please make sure that you have access to the services you use/manage using the non-NUS Atlassian account.
I have other questions, who can I contact?
NUS users can use the the CTLT Helpdesk.
Alumni who were contacted can reply to the email announcement about SSO for Atlassian services.
For Atlassian service-specific support please contact Atlassian Support.