TLS1.0 and TLS1.1 are in the process of being deprecated in Office 365. All clients connecting to Office 365 need to use TLS1.2.
As announced by Microsoft previously, TLS1.0 and TLS1.1 are no longer supported in Office 365 as of October 2018. Customers should no longer be using TLS1.0 and TLS1.1 to connect to Exchange Online. While no date has been set, Microsoft is working towards setting a date for their removal from the service. If either were found to be compromised before then, Microsoft would have no choice but to expedite their removal from Exchange Online.
If your emails are not working then your organization is still using one of those unsupported TLS versions to connect to Exchange Online. Mailboxes are still connecting using SMTP Auth client submission via smtp.office365.com with TLS1.0 or TLS1.1. You can find out which mailboxes by using the SMTP Auth Clients report described in the blog post here.
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