Hello

if you did not upgrade Thunderbird, your network settings and firewall, what is left ? the Internet. More specifically: Office 365.

From (url mangled in the hope of not getting this post going in the moderation queue)

learn.microsoft.com/en-us/microsoft-365/enterprise/ipv6-support?view=o365-worldwide:

• ====(quote)===================================

As with any SaaS service and the Internet overall, the scope of natively IPv6 enabled Microsoft 365 interfaces, features and APIs expands continuously and without direct customer action or control. If you're running IPv6 or IPv6-only services on your networks that need access to Microsoft 365 and the Internet, it is recommended that you include dynamic IPv6/IPv4 transitional mechanisms such as DNS64/NAT64 to ensure end-to-end IPv6 connectivity to Microsoft 365 without any further network reconfigurations.

• =======================================

important to note: 'expands continuously and without direct customer action or control.'

So it may be that the fact that your org did not change the firewall could be the problem. The firewall could have to be updated to reflect the changes in Office 365. IPV6 is in the principle simpler than IPV4, but the devil is in the details, and dual stacks are not simpler than mere IPv4, far from it.

@GP I have updated your answer to make the link actually work. This may in some way be related to the rash of similar issues with hotmail in late December.