Service providers may have some dissatisfied clients as well. I you are a professional service provider and you have received an email filled with complains from your clients, its mandatory that you respond to it in a professional manner. You have to do everything to satisfy your client and try not to repeat such things in the future.
Start drafting your email by stating that you have received the complaints of the recipient and you offer apologies for any discomfort caused from your side.
In the next paragraph, you have to explain why the hitch actually occurred. Brief the client about the root cause of the problem and assure him/her that any such thing won’t happen in the future. Try to satisfy your client as much as you can and even offer some complementary services as well, so that your client remains loyal to you.
The last paragraph of your email should reassure the client that such mistake will not be repeat ever in the future and you will deliver your best to keep your clients satisfied.