Don’t write an apology letter unless you are prepared to take responsibility for whatever went wrong. Say sorry and mean it.
Make sure you use the right name
Check the spelling of your client’s name and their title. (If you can’t be bothered to even get my name right, how much do you really care?)
Lead with “I’m sorry”
“I’d like to apologise” is often used, but “I’m sorry” is more powerful. It’s personal and direct. You don’t need pompous or indirect language to present a professional image; just clear, straightforward communication.
Don’t shift responsibility
When you explain that the delivery was late because the supervisor was off sick, you’re telling them it’s not your fault. (So why are you pretending to apologise?)
“I’m sorry that you …” is not an apology. “I’m sorry that you didn’t get your delivery in time” suggests is that someone else was responsible. Instead, tell them you’re sorry you didn’t get the delivery there on time.
“I’m sorry, but...” is not an apology either. Your client knows that the inevitable excuse is coming, so he stops listening.
Most importantly, don’t blame your client. When you say “I’m sorry that you feel that way” your client hears “I’m not sorry at all. This is entirely your own fault, and I’m only apologising because I have to.”
Tell them your solution
There may be a quick fix, or you may need to have a look at your systems to see what went wrong. Either way, assure your client that you’re taking steps to make sure the problem doesn’t happen again. And be specific if you can. (Vague answers lack credibility.)
Say thank you
Thank your client for their support in the past, and ask for their continued support. Tell them that you value them.
Remember your postscript
The PS is an invaluable tool in business letters. People tend to read the postscript before they read the content of the letter, making it the ideal place to put your key information. But keep it short.
Check your grammar
Every business letter is a reflection on your company. Don’t rely on hazy recollections of high school English lessons; invest in a recognised style guide and make sure your grammar is right.
Every single item of communication with your client is a marketing opportunity; make sure you take full advantage of this. And if you’d rather focus on what you do best, consider outsourcing your business writing to a skilled copywriter.