Problem adding MX records to custom domain - in conflict with Proton Mail

I am trying to add a custom domain to SimpleLogin. I added the required MX records to my registrar, Godaddy, but records not verifying with SimpleLogin.

I already have this same custom domain with Proton Mail. I am not sure how this works. Do I need to set different priorities for the MX records? I can’t find any info within SimpleLogin related to MX conflicts if you already have it setup with Proton Mail.

This is a wild guess, but I think you need different subdomains for each service, like mail.example.com for ProtonMail and spam.example.com for SimpleLogin.

You can only have one custom domain attached to one service. You would need to switch your domain from proton to simplelogin or you need to buy a second domain.