How to check if all emails in the list have one or another domain?

not tested but try something like this.

var text AllEmails = [My semi delimited email list field]);

var text EmailOne= Trim(Part($AllEmails,1,";"));
var user UserTwo =Trim(Part($AllEmails,2,";"));
var user UserThree= Trim(Part($AllEmails,3,";"));

etc

List("\n",

IF(not contains($EmailOne, "X.com") and not contains($EmailOne, "Y.com"), $EmailOne),

IF(not contains($EmailTwo, "X.com") and not contains($EmailTwo, "Y.com"), $EmailTwo),

IF(not contains($EmailThree "X.com") and not contains($EmailThree, "Y.com"), $EmailThree),

etc .. )