Running support: tickets, threading & replies — Reply to the customer

The composer has two tabs. On Reply to customer, To is prefilled with the address the mailbox filed their message into, or failing that the first Email field on the record that holds a value. Type and press Send. The message goes out in your account's branding with your organisation's name as the sender, and it is written into the thread whether or not delivery succeeded. A send that failed is badged Not delivered with the reason — a failed reply that left no trace would tell the next person the customer was never answered, and they would answer twice.