Mail Server: Postfix

A colleague says the mail transfer agent is the whole email server and drops mail into users' inboxes. Where is that wrong, and what actually does the local delivery?

In mail-server terminology, what do MUA, MDA, MTA, and SASL each stand for, and which one is the client program the user actually sees?

A user wants their mail to stay in sync across a laptop, phone, and tablet. Between POP and IMAP, which fits, and what is the core difference that decides it?

You are configuring a mail client and must pick the right port. Which port is for server-to-server SMTP, and which is the submission port that clients should use to send mail?

When storing incoming mail, what is the practical difference between mbox and maildir format, and which one is recommended alongside an IMAP server?

You add an entry to /etc/aliases to redirect root's local mail to your own account, but nothing changes. What step was missed?

Trace the path of an email from the moment a sender clicks Send in their client to the moment the recipient reads it. Which protocol and which program handle each hop?

STARTTLS is described as encrypting a mail connection. At what moment does encryption begin, and what does it deliberately not protect?

Why is a mail server whose relaying is left open the worst configuration mistake you can make, and what is the lasting consequence?

Which DNS record type routes email for a domain, why isn't it an ordinary A record, and what does its priority number decide?

For a mail domain you see CNAME entries like imap, smtp, and pop pointing at the mail host. Are these required for mail to work, and what are they actually for?

To find and resolve a domain's mail servers you run host, but the book warns not to do it on the mail server itself. Which commands do you use, and why run them elsewhere?

Your mail sends fine but recipients keep flagging it as spam. Reverse DNS is mentioned as a defense — what does a PTR entry do and how does its absence hurt you?

You add the reverse DNS entry for your mail server on your domain's DNS control panel, but it never resolves. Why, and where does the PTR entry actually belong?

During a Debian/Ubuntu Postfix install a dialog asks for a configuration type. Which option do you pick for a server that sends and receives mail directly over SMTP?

With a fresh Internet Site install, which accounts can receive mail by default, and what happens when mail arrives for a name that doesn't exist?

On RHEL Postfix is already installed and running, yet external mail never arrives. What is the default limitation, and which main.cf setting lifts it?

On a RHEL mail server, which firewall ports must be reachable, and what firewall-cmd commands open them?

Postfix lookup tables are written as option=type:name with the common type hash. What does a hash table actually read at runtime, and how is that file produced?

In main.cf, what do myhostname, myorigin, mydestination, and relayhost each control, and what value should relayhost have on a standalone server?

On a standalone root server, what must mynetworks contain, and what goes wrong if you make it less restrictive?

You want to see only the Postfix settings you have changed from the defaults, not the whole configuration. Which postconf variant does that, and how do the three variants differ?

Instead of editing main.cf in an editor and reloading, you use postconf -e option=value. What does it do to the file and to Postfix, and what typo silently breaks it?

You're about to change a live mail server's config and worry a mistake will bounce and lose incoming mail. Which setting protects you, and what does it change about error handling?

To let mail clients submit on port 587, which config file do you edit (not the usual one), and what two guarantees does the submission block enforce?

Mail seems stuck and isn't being delivered. Which commands list the queued messages and force an immediate retry, and where does Postfix log its activity?

On a distribution that logs mail through the systemd journal, how do you read just the Postfix messages versus just the Dovecot messages?

In Postfix TLS configuration, what is the difference between the smtpd_ and smtp_ parameter groups, and which one needs your custom certificate?

For receiving mail, what is the practical difference between smtpd_tls_security_level = may and = encrypt, and what does each cost you?

When sending mail, your log shows Untrusted TLS connection. Was the mail encrypted, and which setting turns it into Trusted?

Self-signed certificates are a no-go on public web servers, yet they're considered acceptable on mail servers. What makes the situation different for mail?

On RHEL you generate a self-signed certificate for Postfix that should cover all mail subdomains at once. What must the Common Name look like, and what permission fix is mandatory?

Using a Let's Encrypt certificate for Postfix, which of the issued files must smtpd_tls_cert_file point to, and what must you do after each renewal?

With default Postfix, how much setup does an ordinary Linux user need to become a mail user, and what password do they authenticate with — unlike Samba?

You set home_mailbox in main.cf but Postfix keeps writing mbox instead of maildir. What single character selects the maildir format?

What is a mail alias in /etc/aliases able to forward mail to besides another local account, and what must you run after editing it?

In main.cf you see both alias_database and alias_maps pointing at /etc/aliases. What is the crucial difference between the two parameters?

You want to stop system accounts like daemon or man from receiving mail. Which parameter restricts valid recipients, and what are the steps to apply a custom list?

After editing a mapping file you must rerun postmap, but doing so on a live server risks corrupt lookups. Why, and what is the safe update pattern?

You need addresses like Herbert.Hoover@a-company.com, but Linux usernames can't be that long or contain dots. How do you make it work end to end?

In Postfix terminology, what makes a domain virtual, and what is the simplest way to have one Postfix server host an additional domain?

You host two domains and want hoover@a-company.com and hoover@another-company.com to be different people. Why won't mydestination do it, and what mechanism will?

Virtual mailboxes let Postfix store mail for addresses with no matching Linux account. Who owns those mailbox files, and which parameters configure the setup?

The book warns that virtual mailboxes only pay off under one condition. What is it, and what extra work do they otherwise add?

The SMTP VRFY command checks whether an address exists. Why is that a liability, and how do you turn it off in Postfix?

SMTP authentication seems broken and you want to see what your Postfix server actually offers a connecting client. Which old tool helps, and what in the reply confirms TLS and auth support?

Why is it a serious security mistake to give a sudo-privileged admin account a mail account, even though it's convenient?

You create a Linux account purely to receive mail, no interactive login allowed. What shell do you set, and why does the account still need a real password?