I'm afraid this can not be done easily.
Project access can be regulated with user groups. So if you find a way to automatically put users with a certain domain in separate groups you can achieve your goal.
If you use an external IDP like Azure AD, OneLogin or other and sync the groups to Atlassian Access it will probably be easier.
Otherwise you'll have to use Atlassian's REST API to manage users and groups but the email address of users is not always usable via Atlassian's APIs.
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