Hello. I set new fresh server using Perfect Server based on Ubuntu 18.04 with apache. I would like to know: 1. What is "open relay" mail server? Is that server with all opened ports or something? 2. Can I block port 25 without consequences? 3. Which ports should I open (I am going to use ufw): a) ftp 21 + passive port range configured in pureftp b) www 80, 443 c) pop3 - 110, 995 d) imap - 143, 993 e) smtp - 587, 465, 25 - does this port is used for between servers communication or can be block? f) I am going to have there few node servers, so 3000 - 3010 ports also?