The setup is straightforward, but there are a few pitfalls.
This Full Qualified Domain Name is the full name of the server as <$hostname.$domainname>. When you login to the SMTP server, the server will identify itself with this name
220 <$hostname.$domainname> ESMTP Postfix
This name could be the name as defined in the MX DNS record for your domain. But this is not a requirement.
If your domain is high on the SPAM blacklist, add the name you would get with a reverse DNS lookup. These are not necessarily the same.
- Additional domain.
Add all domains that your server is defined as mail server according to the MX DNS records. This is at least 1 domain.
- SMTP relay
Fill in the SMTP server of your ISP. Note that SSL connection is not supported, only TLS or not-encrypted. Port could be 25, or 587, depending on your ISP. (do NOT use port 465 since that is explicitly used for SSL)