Error in line xrm.SaveChanges(); while updating more than one email record

If I reference a single ActivityId it works fine.

The following error is shown "An error occured while processing this request" at line xrm.SaveChanges();

What am I doing wrong?

        protected void ModifyEmails_Click(object sender, EventArgs e)
        {
            var xrm = new XrmServiceContext("Xrm");

            var GetEmails = xrm.EmailSet
                .Where(c => c.StateCode.Equals(1))
                //.Where(c => c.ActivityId.Equals("e85d7026-0167-e311-b548-3c4a92dbd83d"))
                .Where(c => c.DirectionCode.Equals(true))
                .Where(c => c.mp_contactid2.Equals(null))
                .Take(5)
                .ToList();

            foreach (Email email in GetEmails)
            {
                EntityCollection Recipients = email.GetAttributeValue<EntityCollection>("to");
                ActivityParty ap = Recipients[0].ToEntity<ActivityParty>();
                email.mp_contactid2 = new EntityReference(Contact.EntityLogicalName, ap.PartyId.Id);
                xrm.UpdateObject(email);
            };
            xrm.SaveChanges();
        }