SMTP server setup on Synology [DSM 5.2]

The setup is straightforward, but there are a few pitfalls.

• Hostname(FQDN).
  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)