I bought Poste.io PRO, how do i install full version?
Please continue to installation page.
Why you use single container instead of container per process?
There are couple reasons. It is very easy to distribute running and updating only one image. We can also guarantee that it will work in every conditions. With more containers it is not that easy. Another reason - running mailserver is not that intensive task and mostly will not outgrow single server so we traded full scalability for "easyness to use". For larger installation there is option to separate Antivirus and SpamAssassin which are the greatest consumers of CPU time.
Why SQLite?
It was logical decission since whole mailserver works with files only - emails and settings are files. There is no dependency to running database server. SQLite works with single database file. Vast majority of queries are SELECTS, writes happens only when you add user so it has almost no performance penalty over traditional process based databases. See http://www.sqlite.org/speed.html.
I am getting "550 Relaying denied" when trying to send email
SMTP server is refusing to handle your email because it is not able to identify yourself as user. First and recommended option is to use TLS connection with enabled authentication (port 587 STARTTLS). Second option is to set your IP or subnet as trusted (System settings > Relay settings).