Create sensible subject lines in issue related emails

When our customers write to us via the messenger and we get back to them some time later, they receive an email notifying them of the correspondance. 

They can see exactly what we write to them, but they cannot see the conversation history in the email and thus are confused about what we are writing to them about, especially when they have written to us on multiple occasion for different issues. 

And / or it would be great if the customer can see the whole conversation.