Manually Adding A Domain By Setting Up DNS Records At Cloudflare

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How to setup miab with Cloudflare is discussed here. You need to use the external dns option and manually copy the values that miab suggests to Cloudflare

How to setup with cloudflare and digital ocean

I recommend considering using a domain that is dedicated to your MaiB server, and nothing else. It gives MiaB the freedom to do what the developers want it to do, which is to provide a very reliable standards compliant mail server.