The following dns records is needed before you can add custom domain.
MX record:
[your own domain] 3600 IN MX 10 mail.ddmail.se.
SPF as TXT record:
[your own domain] 3600 IN TXT "v=spf1 mx -all"
DKIM as TXT record:
mail._domainkey.[your own domain] 3600 IN TXT "v=DKIM1; k=rsa; \009p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEApJLX14mun5DlSlcqlJLv1c4eLIbtPxzhuFl5ZI5IUS5ByI6eE/5zHLrdt3XmtTAjM6xmdE4DOn7Un8SigJCcTQHkUktHF3E269qXn3lc0dyBl8BlC7e+WzPoVsVLcdCbHM+ZIgjSJ+7rHnD3zM0AysQaI+N72Rg+sCXLW9KC5BYCcecdtzfJg68+Jq5COuC" "IHEwD1QLyFgqvSW9X5 \009YX2BLRqzrsSqaGet4jEF1x/b/9VWkw8HptbzF0BDSpEin49Jakeiz4pJqtrnu95LuJwEDcHEFe5oNTmTksDd6RGWBbCMQ3WTA1QhANYVpPtL+BxUKGELE6nT0zntVi5wtxYjwIDAQAB"
DMARC as TXT record:
_dmarc.[your own domain] 3600 IN TXT "v=DMARC1; p=none"