are a couple of ways in which mailing houses assert their quality issues
ISO9000 certification. This is the European standard of quality assurance. Unfortunately there are one or two rogue firms that assert they have this, when in fact all they have done is purchased it from an unlicensed body. A copy that has DTI recognised ISO9000 certification really is getting its procedures in order.
Trade association membership. You will have to judge what benefit you think membership of a trade association gives you, as opposed to the benefit it gives to the company you are dealing with. While it is true that absolute rogue companies would be thrown out of most associations, it is also true that many associations do very little to check the activities of their members – unless a complaint comes along.