Keep the original subject on replies
When you answer a ticket, leave the customer's subject as it is with the usual Re: prefix. Changing it breaks the thread in their mail app and makes your answer look like a new message they can miss.
For emails you start, name the topic and the status
Say what the message is about and whether it needs anything from them. Specific beats clever every time.
- Your refund for order 1042 has been issued
- Quick question about the export error you reported
- Your request is fixed: please confirm
- We need one more detail to look into your invoice
Put the ask first when there is one
If you need the customer to do something, the subject should say so, so it stands out from routine updates. Keep the action short and put the reference after it.
Avoid the ones that read as noise
Vague subjects such as Update or Following up, all caps, and urgent markers on routine mail train customers to ignore you. A subject with no reference to their problem looks like marketing.
Keep it short enough for a phone
Many people read on a phone, where only the first 40 or so characters show. Put the topic and the action in that space and the order number after it.