What to write after you apologize
You received an email from one of your customers about a legitimate problem and you've written a sincere apology. That's great, but the I'm sorry may not be all you need to say. What you write after the apology can be just as important as the apology itself. You can explain why the problem happened, or you might want to explain what you're going to do to fix the problem, or it might be best to explain what the customer can do if this happens again. Use any one or a combination of these explanations in your reply.
Let's look at an example. A customer has written your company an angry email because she was locked out of her online account. No matter what she tried she couldn't log in. And why was she locked out? Because your system had a day long glitch and all your customers were unable to log in. It was a nightmare, but now it's fixed. In this situation you'll definitely have to write a sentence or two of sincere apology, then choose which of the three approaches is best suited to the situation.
Source: https://www.linkedin.com/learning/writing-customer-service-emails/what-to-write-after-you-apologize