Re: (Incident Here)
1st Paragraph – An Introduction. This should state the reason for the apology, and should indicate that you understand the customer complaint and that you are in agreement with the customer and you are sorry.
2nd Paragraph – Find a solution. Go on to state what you intend to do in order to rectify the problem that has arisen, whether this will be to change a system, or to offer something to make up for the problems caused in the first place.
3rd Paragraph – Conclusion. This paragraph needs to restate how sorry you are for the problems that you/your company has caused, and again you should restate what you are doing to resolve the situation. You can also add your belief that the problem will not occur again in future.