I always test emails with MailDev. Just a quick npm install -g maildev and you have an SMTP server that won’t actually send your emails anywhere. Instead it’ll collect all of the emails that your application tries to send and give you an inbox to look at them.
With Kirby I use localhost config to set MailDev as the SMTP server for my dev environment and the production config file to set the production SMTP server.
I posted a bit more info about how my dev environment and deployment fit together in this thread.