I want to use Office365 for only a limited number of email addresses associated with my own domain. Is this possible? Will legacy email clients still work on other email addresses?
Based on your description, I suggest you post this issue to Microsoft 365 admin forum:
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
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