And this is where "cloud services" fall flat on its face.
Face it, you're at the mercy of your ISP once you've outsourced every.single.server (including Exchange and file storage) to the cloud - and if your ISP should go TITSUP* there's nothing you can do.
Except wait for services to be restored.
Should your services be on-prem, your users can still work on their spreadsheets, documents and so on, but emails (should you also be wise and have an on-prem Exchange/Linux email server) will simply spool and wait for a chance to escape the server's clutches.
However, in both cases with emails, you won't be able to receive - unless your workforce is equipped with mobile phones, which (hopefully) is working and you can get access to emails that way.
Next time some beancountery type suggests going cloud-based, have them factor in a two-three day full internet outage into their calculations.