IEnumerable<GroupEmailCust> eMailAll = GetGroup();
            IEnumerable<GroupEmailCust> eMailGrp = eMailAll.Where(c => c.CustomGroupID == id);
            IEnumerable<GroupEmailCust> eMailNotGrp = eMailAll;

            foreach (var cust in eMailGrp)
            {
                eMailNotGrp = eMailNotGrp.Where(g => g.EMailID != cust.EMailID).OrderBy(c=>c.Custom);
            }
            return eMailNotGrp;

       public IEnumerable<GroupEmailCust> GetGroup()
        {
            var eMail = from b in db.bCustoms
                        join m in db.EMails
                            on b.CustomID equals m.CustomID
                        join g in db.MailToGroups
                            on m.EMailID equals g.EMailID into t1
                        from g1 in t1.DefaultIfEmpty()
                        where m.ismark==0 
                        select new GroupEmailCust
                        {
                            EMailID = m.EMailID,
                            CustomID = m.CustomID,
                            Custom = b.txt,
                            Mail = m.Mail,
                            Name = m.Name,
                            CustomGroupID = g1.CustomGroupID
                        };
            return eMail;
        }

   public partial class bCustom //Контрагенты
    {
        public int CustomID { get; set; }
        public string txt { get; set; }
    }

    public partial class EMai l//Адреса. У контрагента может быть несколько. Не принципиально в данном случае
    {
        public int EMailID { get; set; }
        public int CustomID { get; set; }
        public string Mail { get; set; }
        public string Name { get; set; }
    }

    public partial class CustomGroup //Группы рассылки
    {
        public int CustomGroupID { get; set; }
        public string Name { get; set; }
    }

   public  class MailToGroup //связь Группы-Адреса
    {
        public int MailToGroupID { get; set; }
        public int EMailID { get; set; }
        public int CustomGroupID { get; set; }
    }

  public class EmailCust
    {
        [Key]
        public int EMailID { get; set; }
        public int CustomID { get; set; }
        [Display(Name = "Контрагент")]
        public string Custom { get; set; }
        [Display(Name = "Email")]
        public string Mail { get; set; }
        [Display(Name = "Контактное лицо")]
        public string Name { get; set; }
    }

    public class GroupEmailCust:EmailCust
    {
        public int? CustomGroupID { get; set; }
    }

var query = from m in AllMails
                 where !MailToGroups.Any(x => x.EMailID == m.EMailID && g.CustomGroupID != 1)
                 select m;