RelatedToId field on EmailMessage Not Writeable

Question

I am getting this response 'Field is not writeable: EmailMessage.RelatedToId'

Email trigger helper method is the following:

public static void parseEmailSubjectForLeadRelation(List<EmailMessage> emailList){
        system.debug('Entered parseEmailSubjectForLeadRelation');
        List<EmailMessage> emList = new List<EmailMessage>();
        string leadId;
        for(EmailMessage email : emailList) {
            system.debug('email == ' + email.TextBody + ' / ' + email.ParentId);
            if(email.Incoming == true && email.ParentId != null && email.TextBody != null) {
                if(email.TextBody.contains('reference:')){
                    email.RelatedToId = email.TextBody.substringBetween('reference:{', '}');
                    emList.add(email);
                }
            }
        }
        update emList;
}

This is being called on AfterInsert. I have seen another similar question that was fixed by updating the api version of the apex class which i have already tried. I have also double checked that enhanced email is enabled in this org.

Answer 1

According to the documentation for the EmailMessage Object, this field can only be set on insert not update