Focus on fixing the problem and being transparent. Tell them how they can reach out to you again. You must also keep your messages free from grammatical errors — consumers will not trust a consultant who does not write well.
Writing customer service emails that feel personal is an important part of the job. That feature already exists. Remain friendly in every situation, and your very best to stay polite and courteous, even if your client becomes aggressive or offensive.
Even the most subtle of shifts in phrasing can have a huge impact on how you make the customer feel. Let them know when you will be contacting them again with an update.