This gets a little tricky.
What you can try to do is pull the RDS connection string for the Metabase application db, connect directly and insert email creds into the settings db. It's a little hokey but we currently don't have another way to reset admin passwords.
You'd want to insert credentials in the following form:
email-smtp-host : hostname
email-smtp-username : username
email-smtp-password : password
email-smtp-port : port you should use to connect
email-smtp-security : ssl | tls | don't insert
email-from-address : a white listed email sender on your service