Set Task.Type on Enhanced Email Activities

EmailTaskDefaultType.trigger

trigger EmailTaskDefaultType on EmailMessage (after insert, after update) {
    static final String EMAIL_TASK_TYPE_DEFAULT = 'Email';

    List<Task> toUpdate = new List<Task>();

    for (EmailMessage newEmailMessage : Trigger.new) {
        if (newEmailMessage.ActivityId != null) {
        
            EmailMessage oldEmailMessage = null;
            if (Trigger.isUpdate) {
                oldEmailMessage = Trigger.oldMap.get(newEmailMessage.Id);
            }

            if (oldEmailMessage == null || oldEmailMessage.ActivityId == null) {
                Task t = new Task(Id=newEmailMessage.ActivityId);
                t.Type = EMAIL_TASK_TYPE_DEFAULT;
                toUpdate.add(t);
            }
        }
    }
    
    update toUpdate;
}

EmailTaskDefaultType.trigger-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<ApexTrigger xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>47.0</apiVersion>
    <status>Active</status>
</ApexTrigger>

EmailTaskDefaultTypeTriggerTest.cls

@IsTest
private class EmailTaskDefaultTypeTriggerTest {
    @IsTest
    static void thatRelationsAddedAfterInsertAreIncluded() {

        Lead l = new Lead(FirstName='Test', LastName='Lead', Company='ACME Testing', Email='test@example.com');
        insert l;

        EmailMessage createdWithoutActivity = new EmailMessage(Subject='test subject', TextBody='test body', Status='3');
        insert createdWithoutActivity;

        EmailMessageRelation rel = new EmailMessageRelation();
        rel.EmailMessageId = createdWithoutActivity.Id;
        rel.RelationId = l.Id; // user id of the sender
        rel.RelationType = 'ToAddress';
        insert rel;

        Test.startTest();

        EmailMessage afterInsert = [SELECT Id, ActivityId FROM EmailMessage WHERE Id = :createdWithoutActivity.Id];

        Id taskId = afterInsert.ActivityId;
        Task t = [SELECT Id, Type, TaskSubtype FROM Task WHERE Id = :taskId];

        System.assertEquals('Email', t.Type);
        System.assertEquals('Email', t.TaskSubtype);

        Test.stopTest();
    }


}