Users may be unable to send email to outlook.com (portal.office.com)

Not sure if it's relevant, but I also see DNSSEC issues:

https://dnssec-analyzer.verisignlabs.com/outlook.com

The same applies to all domains using Office 365 for emails. DNS lookups seem to time out most of the time, but occasional ones get through. Seems to be affecting EMEA more than other regions.

Same issue since April 20?? Atlassian-level downtime