Electronic Mail, Types of MUA and MTA, and Sending Email in Linux

Email is one of the oldest and still most heavily used applications on the Internet, predating the World Wide Web itself. Behind every email you send or receive is a coordinated system of specialized software components — Mail User Agents, Mail Transfer Agents, and Mail Delivery Agents — working together across protocols like SMTP, IMAP, and POP3. This article explains, from first principles, how email actually works, the different software roles involved, and how to send email from a Linux system, both interactively and programmatically.

The Building Blocks of Email

flowchart LR
    A[Sender's MUA] --> B[Sender's MTA]
    B --> C[Recipient's MTA]
    C --> D[MDA]
    D --> E[Recipient's Mailbox]
    E --> F[Recipient's MUA - IMAP/POP3]

MUA — Mail User Agent

The MUA is the software a human actually interacts with to read and compose email. Examples:

• Graphical MUAs: Thunderbird, Outlook, Apple Mail, Gmail’s web interface
• Command-line MUAs (Linux): mutt, mail/mailx, alpine

MTA — Mail Transfer Agent

The MTA is server software responsible for routing and relaying email between servers using SMTP (Simple Mail Transfer Protocol). Examples:

• Postfix (most common on modern Linux servers)
• Exim (default on many Debian-based systems)
• Sendmail (historic, still found on legacy systems)

MDA — Mail Delivery Agent

The MDA takes an incoming message from the MTA and actually delivers it into a specific user’s mailbox, sometimes applying filtering rules along the way. Examples:

• Dovecot (also commonly serves as an IMAP/POP3 server)
• Procmail (classic filtering MDA)

Retrieval Protocols: IMAP and POP3

Once mail is delivered to a mailbox, the recipient’s MUA retrieves it using either:

• IMAP (Internet Message Access Protocol) — mail stays on the server, synchronized across multiple devices (the modern standard)
• POP3 (Post Office Protocol v3) — mail is downloaded to a single device, often removed from the server afterward (largely legacy today)

Comparison Table: Email Component Roles

The Full Journey of an Email

Imagine Alice (alice@companyA.com) sends an email to Bob (bob@companyB.com):

sequenceDiagram
    participant Alice as Alice's MUA
    participant MTA_A as companyA MTA
    participant DNS
    participant MTA_B as companyB MTA
    participant Bob as Bob's MUA

    Alice->>MTA_A: SMTP: Send message
    MTA_A->>DNS: MX record lookup for companyB.com
    DNS-->>MTA_A: mail.companyB.com
    MTA_A->>MTA_B: SMTP: Relay message
    MTA_B->>MTA_B: MDA delivers to Bob's mailbox
    Bob->>MTA_B: IMAP: Fetch new messages

1. Alice’s MUA sends the message to her organization’s outgoing MTA via SMTP.
2. That MTA looks up companyB.com’s MX (Mail Exchange) DNS record to find the responsible mail server.
3. The sending MTA relays the message to the receiving MTA over SMTP (typically port 25 between servers).
4. The receiving MTA hands the message to an MDA, which delivers it into Bob’s mailbox.
5. Bob’s MUA later retrieves the message using IMAP (or POP3).

Checking MX Records (DNS for Mail Routing)

dig MX companyB.com

dig MX companyB.com

Example output:

companyB.com.  3600  IN  MX  10 mail.companyB.com.

companyB.com.  3600  IN  MX  10 mail.companyB.com.

The number (10) is a priority value — lower numbers are preferred when multiple MX records exist.

Sending Email From the Linux Command Line

Using mail (mailx)

echo "This is the message body." | mail -s "Test Subject" recipient@example.com

echo "This is the message body." | mail -s "Test Subject" recipient@example.com

Install if not present:

sudo apt install mailutils          # Debian/Ubuntu
sudo dnf install mailx              # RHEL/Fedora

sudo apt install mailutils          # Debian/Ubuntu
sudo dnf install mailx              # RHEL/Fedora

Sending With Attachments

echo "See attached report." | mail -s "Weekly Report" -A report.pdf recipient@example.com

echo "See attached report." | mail -s "Weekly Report" -A report.pdf recipient@example.com

Using sendmail Directly

sendmail recipient@example.com <<EOF
Subject: Direct Sendmail Test
From: admin@myserver.com

This message was sent using the sendmail command directly.
EOF

sendmail recipient@example.com <<EOF
Subject: Direct Sendmail Test
From: admin@myserver.com

This message was sent using the sendmail command directly.
EOF

Using mutt for Interactive Use

mutt -s "Subject line" recipient@example.com

mutt -s "Subject line" recipient@example.com

mutt opens your configured editor to compose the body, and supports attachments, address books, and IMAP/SMTP account configuration for a full terminal-based email experience.

Sending Email Programmatically With Python

import smtplib
from email.mime.text import MIMEText

msg = MIMEText("This is the body of the email.")
msg['Subject'] = 'Automated Report'
msg['From'] = 'reports@example.com'
msg['To'] = 'admin@example.com'

with smtplib.SMTP('smtp.example.com', 587) as server:
    server.starttls()
    server.login('reports@example.com', 'app-specific-password')
    server.send_message(msg)

print("Email sent successfully.")

import smtplib
from email.mime.text import MIMEText

msg = MIMEText("This is the body of the email.")
msg['Subject'] = 'Automated Report'
msg['From'] = 'reports@example.com'
msg['To'] = 'admin@example.com'

with smtplib.SMTP('smtp.example.com', 587) as server:
    server.starttls()
    server.login('reports@example.com', 'app-specific-password')
    server.send_message(msg)

print("Email sent successfully.")

This is a common pattern for server monitoring scripts that email alerts automatically.

Configuring Postfix as a Basic Outgoing MTA

For a server that just needs to send outgoing notification emails (not receive), a minimal Postfix “satellite” configuration works well.

Step 1: Install Postfix

sudo apt install postfix

sudo apt install postfix

During installation, choose “Internet Site” or “Satellite system” depending on your setup.

Step 2: Key settings in /etc/postfix/main.cf

myhostname = mail.example.com
relayhost = [smtp.relay-provider.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt

myhostname = mail.example.com
relayhost = [smtp.relay-provider.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt

Step 3: Configure SASL credentials for the relay

echo "[smtp.relay-provider.com]:587 username:password" | sudo tee /etc/postfix/sasl_passwd
sudo postmap /etc/postfix/sasl_passwd
sudo chmod 600 /etc/postfix/sasl_passwd*

echo "[smtp.relay-provider.com]:587 username:password" | sudo tee /etc/postfix/sasl_passwd
sudo postmap /etc/postfix/sasl_passwd
sudo chmod 600 /etc/postfix/sasl_passwd*

Step 4: Restart Postfix

sudo systemctl restart postfix

sudo systemctl restart postfix

Step 5: Test

echo "Test message" | mail -s "Postfix relay test" you@example.com

echo "Test message" | mail -s "Postfix relay test" you@example.com

Real-World Use Case: Automated Backup Notification

#!/bin/bash
BACKUP_LOG="/var/log/backup.log"
if tar -czf /backups/data-$(date +%F).tar.gz /data > "$BACKUP_LOG" 2>&1; then
    echo "Backup completed successfully on $(date)" | mail -s "Backup Success" admin@example.com
else
    cat "$BACKUP_LOG" | mail -s "Backup FAILED" admin@example.com
fi

#!/bin/bash
BACKUP_LOG="/var/log/backup.log"
if tar -czf /backups/data-$(date +%F).tar.gz /data > "$BACKUP_LOG" 2>&1; then
    echo "Backup completed successfully on $(date)" | mail -s "Backup Success" admin@example.com
else
    cat "$BACKUP_LOG" | mail -s "Backup FAILED" admin@example.com
fi

Comparison: SMTP vs. IMAP vs. POP3

Best Practices

• Always use authenticated, encrypted submission (port 587 with STARTTLS) rather than legacy unauthenticated relaying on port 25, which is heavily blocked and abused.
• Set up SPF, DKIM, and DMARC DNS records for any domain sending mail, to improve deliverability and reduce the chance of being marked as spam.
• Use a reputable relay/SMTP provider for application-generated email (transactional email services) rather than running your own outbound mail server from scratch, unless you have the expertise to manage IP reputation.
• Never hard-code SMTP credentials in scripts — use environment variables, secret managers, or app-specific passwords with limited scope.
• Monitor mail queues (postqueue -p for Postfix) to catch delivery failures early.
• Rate-limit automated email alerts to avoid flooding inboxes during incident storms.

Troubleshooting

Problem: Email sent but never arrives

Check the mail queue and logs:

postqueue -p
sudo tail -f /var/log/mail.log

postqueue -p
sudo tail -f /var/log/mail.log

Problem: Relay access denied

The MTA isn’t configured to relay for your domain/IP. Check mynetworks and relay authentication settings in Postfix’s main.cf.

Problem: Mail marked as spam by recipients

Verify SPF, DKIM, and DMARC records are correctly configured:

dig TXT example.com
dig TXT _dmarc.example.com

dig TXT example.com
dig TXT _dmarc.example.com

Problem: mail command not found

Install the mail utilities package:

sudo apt install mailutils

sudo apt install mailutils

Problem: Python smtplib connection times out

Check that the SMTP port isn’t blocked by a firewall or your ISP (many residential/cloud ISPs block outbound port 25 by default; use 587 instead):

telnet smtp.example.com 587

telnet smtp.example.com 587

Conclusion

Email is a distributed system built from clearly separated roles — the MUA you interact with, the MTA that routes messages between servers, and the MDA/retrieval protocols (IMAP/POP3) that get mail into your inbox. Understanding this architecture, along with practical Linux tools like mail, sendmail, mutt, Postfix, and Python’s smtplib, allows you to both use email effectively as a system administrator and build reliable automated notification systems for scripts, monitoring, and backups.

Further Reading

• RFC 5321 — Simple Mail Transfer Protocol
• RFC 3501 — Internet Message Access Protocol
• Postfix documentation
• Python smtplib documentation
• DMARC.org
• Arch Wiki: Postfix