"We offer a complete migration service that includes setting up an Office 365 account and users, rebuilding the client website in Office 365, and managing the redirection of Web and email traffic to Office 365, working with the domain registrar as necessary. Clients may also retain us to perform specific aspects of the migration," he said.

The extent of the fallout will become clearer on Tuesday and on the days after. If the number of affected customers is significant, the consequences for them and for Microsoft could be very costly.

Even if their email continues to work for six more months, small businesses that find their website is suddenly offline will inevitably be harmed by the situation, since websites have become essential in sales and marketing efforts.

For Microsoft, it would be a black eye as it battles Google and others providers of cloud-based communication and collaboration suites with its new Office 365.

The from OLSB customers about the decision to close the service began many months ago and have grown in intensity as the deadline approached.