Invalid type: Schema.EmailMessageRelation

Question

Im receiving

Invalid type: Schema.EmailMessageRelation

and

Invalid type: EmailMessageRelation

compiler error while saving the code.

public class twilioSmsTriggerHandler {
    public twilioSmsTriggerHandler() {

    }
    list<Id> emailIdList = new list<Id>();

    public void newEmails(List<EmailMessage> emails) {
        for(EmailMessage emailId : emails) {            
            emailIdList.add(emailId.Id);
        }

        for(EmailMessageRelation emailmsgId : [SELECT EmailMessageId,
                                                      RelationAddress,
                                                      RelationId,
                                                      RelationObjectType,
                                                      RelationType
                                               FROM EmailMessageRelation
                                               WHERE EmailMessageId =: emailIdList]) {  

        system.debug(emailmsgId);
    }

    }
}

Answer 1

I'm moving this comment to an answer because I don't see that the question has been clarified.

The SOAP Reference for EmailMessageRelation states:

EmailMessageRelation is only available for organizations that use Email-to-Case or Enhanced Email, which is automatically enabled for most customers.

Further, the object is available only in API 37 and up.

It's not likely that your org doesn't have Enhanced Email turned on, but it's certainly possible. You should also confirm that the class you are working on is on at least API version 37.0, and preferably more recent.