We've got a Windows Service which sends emails using EWS API and is using Basic Authentication for that. As well as that it uses EWS API to check some folder in the mail box to retrieve certain email which is going to be used as template. To be honest I'm not pro in mail-servers and not sure if retirement of basic auth approach will affect us.
Another thing to consider is that our team doesn't have control over Azure tenants and it's our client's IT department who has it and they are going to create new Azure tenant for that and as far as I understood by default basic auth will be disabled for it.
So should we updated our service to Graph API in this case or would it be possible somehow to reenable basic auth for that new tenant? Thanks!
The Change to Basic Authentication has been deferred now to 2021 rather then October https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-and-exchange-online-april-2020-update/ba-p/1275508
It sounds like you don't quite understand how Office365 and AzureAD work as most of what your saying is incorrect. The only thing you need for oAuth is an Azure Application registration https://learn.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app . If your app is going to be used by lot of different clients then make it multi-tenant. What each client would need to do is Grant Consent for your application to be used in their tenant https://learn.microsoft.com/en-us/azure/active-directory/manage-apps/grant-admin-consent (this is just a one off thing).
If you use the Graph API you will still need the same registration/consent to use oAuth