Please help me with a trigger to populate contact's email domain in Account custom field seperated by comma.

I created a custom field in Account object 'Contact_Email__c' which is long text area type. you can use below code.

//Trigger
trigger UpdateEmail on Contact(before insert, after update) {
   if((Trigger.isBefore && Trigger.isInsert) || (Trigger.isUpdate && Trigger.isAfter)){
        UpdateEmailClass.updateEmailMethod(trigger.new);
   }
}
//Trigger helper class
public class UpdateEmailClass{
    public static void  updateEmailMethod(List<Contact> contactList){
        Set<Id> AccIdSet = new Set<Id>();
        Map<Id,List<String>> accIdVsString = new  Map<Id,List<String>>();
        for(Contact conObj : contactList){
            if(conObj.AccountId != NULL && conObj.Email != NULL){
                AccIdSet.add(conObj.AccountId);
                if(!accIdVsString.containsKey(conObj.AccountId)){
                     accIdVsString.put(conObj.AccountId, new List<String>());
                }
               accIdVsString.get(conObj.AccountId).add(conObj.Email);
            }
        }
        List<Account> accountList = new List<Account>();
        if(AccIdSet.size() > 0){
            accountList = [SELECT Name, Contact_Email__c FROM Account WHERE Id IN: AccIdSet LIMIT 10000];
        }
        if(accountList.size() > 0){
            for(Account accObj : accountList){
                    if(accIdVsString.containsKey(accObj.Id)){
                        accObj.Contact_Email__c = accObj.Contact_Email__c+', '+accIdVsString.get(accObj.Id);
                    }
            }
        }
        update accountList;
    }
}

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