Hi there,
I found that was the case with me too. Doesn’t appear in the dropdown, but was able to enter the details in the settings form.
Under Type, choose SMTP.
Under Outgoing Mail Server Hostname, add your AWS mail server address
Under Security, choose STARTTLS
Under Authentication, choose the plain option and add your AWS_ACCESS_KEY_ID to Username and AWS_SECRET_ACCESS_KEY to Password. Worked for me!