warning: do not list domain example.com in BOTH mydestination and virtual_mailbox_domains
Yes, thats the most likely reason for the problem. You may not use a domain as mydestination, mydomain, etc. in a postfix virtual setup that is used as virtual mail domain too. Thats why the perfect setup tutorial instructed you to use server1.example.com and not example.com as hostname for the server.
To fix that, edit /etc/postfix/main.cf and replace all occurrences of domain.tld with something like server1.domain.tld and restart postfix. Then create server1.domain.tld as dns a-record in the zone domain.tld a dnpoint it to the Ip address of your server.