Do you want to replace a Microsoft e-mail client, e.g., Outlook on Windows, or do you want to replace a Microsoft account or both? You cannot switch from outlook.office.com to Thunderbird. You can switch from accessing your outlook.office.com account in one e-mail client, e.g., Outlook (or webmail), to a different e-mail client, e.g., Thunderbird. And you can switch from using outlook.office.com to using a different account from a different e-mail service provider, e.g., Gmail. If you want to replace a Microsoft e-mail client with Thunderbird, that is easy, but transferring messages that were saved locally in your old e-mail client will require some effort and maybe expense. If you want to replace your Microsoft 365 account, you will have to establish a new account with some e-mail service provider, then set up the new account in Thunderbird. And you would need a plan for transferring the messages from the old account to the new account. Where the domain is hosted is independent or other considerations. My domain and my e-mail are hosted by different services. Regardless of what you do with e-mail clients, your business e-mail account should always work in webmail. "Email" has come to mean three different things. I do not know if you are asking about accounts or messages. You can move one account at a time on your own schedule. I think that's what you wanted to know.