To run a successful business, you need to keep your customers always satisfied. You might receive customer complaints regarding the products you sale to them or the services you provide and it is your utmost obligation to adhere to those customer complaints. Writing an apology email to the customers shows your professionalism and the care you have for your customers.
Begin the email with apologizing the customer about whatever went wrong. Tell the recipient how much you value your customers and you are indeed very sorry for causing the inconvenience to them.
In the second paragraph of your email, you better explain the cause of the hitch to your customer. Accept that it was your company's fault and actions will be taken against those responsible. Assure your customer that such problem will never happen in the future.
In the third paragraph, you should assure your customer that every necessary step will be taken to make things right for him/her. give the solution to the problem and insist your customer to remain loyal to your organisation in the future as well.