Outgoing Mail Queue
Every message the server has accepted for sending and not delivered yet, across all accounts. The states of a message, who sent it, how fresh the list is, and what limits how fast mail enters the queue.
Outgoing Mail Quarantine
The server holds the outgoing mail of a mailbox or send-only user whose activity looks like a stolen password. The automatic rules and their defaults, the server-wide settings, what you can do with the held mail, how long it is kept, and the host:account:quarantine commands.
Email Traffic Log
The delivery record of every message the server handled, across all accounts. How to find one message from a queue ID or delivery ID, how the records reach the panel, and how long they are kept.
Relay to an account holder's own server
A domain's MX points at the server, but some of its addresses live on a mail server the account holder runs. The service:mail:relay commands that set up, inspect and repair that relay, and where the outside gateways in front of or behind a domain are set.
System mail
Where root's mail, the mail system's own notices, the server's abuse reports and the panel's notices to you land, how --admin-email forwards them to you, and how postmaster@ and abuse@ are answered on every domain.
Virus scanning
ClamAV as an optional add-on to the spam filter, on incoming and outgoing mail. What installing it turns on, what a detection does, why it fails open, what it costs in memory, and how to check it is working.