There is no preview URL, you will need to change the nameservers.
Technically, more experienced users could modify your local hosts file to force your machine to use the x10 server IP when going to your domain.
I asked an admin but I do not think it will be possible as it becomes an internal pointer for ACME on the servers themselves.
I would use the DNS method if possible.
You will need to attempt renewal manually through Account Manager > SSL Certificates > Get automatic certificate from ACME Provider.
If you get a rate limit error, you will either need to patiently wait for limit to reset or use alternative sites like sslforfree to generate one and manually...
Using PHP mail() is unauthenticated.
You are best to use a library like PHPMailer which has support for authentication to log into the mailbox to send mail as that mail address.
3 other things are either missing or not properly configured.
1. Enable DKIM
Email Accounts > Enable DKIM
2. SPF...