We have SharePoint O365 Tenant.
We are using Microsoft.SharePoint.Client.Utilities.Utility.SendEmail to send emails to users through a Console Application.
However, it does not send to some external users. It is weird because some external users are able to receive emails. They are using the same email domain (ex. firstname.lastname@example.org). External users are also using SharePoint Online O365 too.
When debugging, we noticed that those that do not receive email are missing the value for the EMail field in SiteUserInfoList. Those that have the value receive emails. We checked the User Profiles Work Email value for the users, and they are correct.
Are there any particular settings we need to do to fix the issue?