Explain the use of SMTP-Relay services

This package warns against setting up E-Mail servers on residential connection and not being able to setup a rDNS or static IPs. There exists another option, which is the use of SMTP-Relay services. This package then takes care of receiving E-Mail, including spam filtering and anti-virus checks, but not sending.

I added a passage to explain this possibility. This takes the worry of your E-Mails landing in Spam due to having no static IP, having a static IP but with a blacklisted history and/or being unable to setup rDNS.