The Only SMTP Guide You’ll Ever Need (From Ports to Production)

1. What Is SMTP in Plain English?

2. SMTP vs IMAP vs POP3: Stop the Confusion

3. How SMTP Works Under the Hood (With Real Packet Trace)

1. Handshake
   Your client opens a TCP connection to the server on port 587.
   220 mail.trashmail.in ESMTP Postfix
2. EHLO
   Client says hello with its name:
   EHLO laptop.local
3. STARTTLS
   Connection upgrades to TLS. No credentials float in plain text.
4. AUTH LOGIN
   Base64-encoded username and password swap hands.
5. MAIL FROM
   MAIL FROM:<priya@trashmail.in>
6. RCPT TO
   RCPT TO:<rohan@example.com>
7. DATA
   Headers + body transmitted. One dot on its own line ends the message.
8. QUIT
   Connection closes. Rohan’s server now accepts or rejects the payload.

4. The Three SMTP Ports Explained (And Which One Google Actually Likes)

5. Setting Up Your Own SMTP Server on Trashmail.in (Step-by-Step)

• Ubuntu 22.04 LTS
• Trashmail.in domain with DNS control
• 2 GB RAM (handles 50 k emails/day comfortably)

sudo apt update && sudo apt install postfix libsasl2-2 sasl2-bin

sales@trashmail.in    salesuser
support@trashmail.in  supportuser

sudo postmap /etc/postfix/virtual

submission inet n       -       y       -       -       smtpd
  -o syslog_name=postfix/submission
  -o smtpd_tls_security_level=encrypt
  -o smtpd_sasl_auth_enable=yes

sudo certbot certonly --standalone -d mail.trashmail.in

v=spf1 mx ~all

swaks --to rohan@gmail.com --from priya@trashmail.in --server mail.trashmail.in:587 -tls -au priya -ap 'SuperSecret'

6. Common SMTP Response Codes & How to Fix Them Fast

7. Authentication Deep Dive: CRAM-MD5, OAuth2, and SCRAM

import base64
def xoauth2_string(user, access_token):
    return base64.b64encode(
        f'user={user}\x01auth=Bearer {access_token}\x01\x01'.encode()
    ).decode()

8. Deliverability Secrets Nobody Tells You

1. Warm-up IP gradually—start with 50 messages/hour, double daily.
2. Keep spam complaint rate <0.1 %. Use List-Unsubscribe header.
3. Send at consistent volumes; sudden spikes trigger rate limits.
4. Prune hard bounces within 24 h; they’re poison to reputation.
5. Use TLS-RPT to catch TLS downgrade attacks.

9. Troubleshooting Toolbox

• swaks – Swiss Army Knife; tests every SMTP verb.
• mxtoolbox.com – Checks 120 RBLs in one click.
• mail-tester.com – Gives your message a spam score out of 10.
• tcpdump – When you absolutely need to see every packet.
• Postfix qshape – Visual queue age distribution.

10. Advanced: Combining SMTP with Webhooks for Real-Time Alerts

#!/usr/bin/env python3
import sys, requests
recipient = sys.stdin.readline().split('=')[1]
requests.post('https://hooks.slack.com/xxx',
              json={'text': f'Bounce from {recipient}'})

bouncehook unix  -       n       n       -       -       pipe
  flags=F user=nobody argv=/usr/local/bin/bouncehook.py

11. Security Checklist (Print & Pin)

12. Scaling Beyond One Server

                   +-----------+
                   |  NGINX    |  (TCP load balancer)
                   +-----+-----+
                         |
       +-----------------+------------------+
       |                 |                  |
+------+------+  +-------+-------+  +-------+-------+
| SMTP-1      |  | SMTP-2        |  | SMTP-3        |
| 8 GB RAM    |  | 8 GB RAM      |  | 8 GB RAM      |
+------+------+  +-------+-------+  +-------+-------+
       |                 |                  |
+------+-----------------+------------------+
| Shared MariaDB (queue, quotas, logs)
+-------------------------------------------+

13. Green SMTP: Cutting Carbon Per Email

• Choose SMTP hosts powered by renewables.
• Strip 20 kB images; saves 0.8 g per message.
• Batch newsletters weekly instead of daily.
• Use BIMI SVG logos (lighter than PNG).
  Trashmail.in switched to a hydro-powered DC and cut annual emissions by 12 t—equivalent to 2 800 cars off the road for a day.

14. Future-Proofing: SMTP over QUIC?

15. Cheat-Sheet: Copy-Paste Configs

myhostname = mail.trashmail.in
myorigin = trashmail.in
mydestination = $myhostname, localhost.$mydomain, localhost, trashmail.in
virtual_alias_maps = hash:/etc/postfix/virtual
smtpd_tls_cert_file = /etc/letsencrypt/live/mail.trashmail.in/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/mail.trashmail.in/privkey.pem
smtpd_use_tls=yes
smtpd_tls_security_level = may
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes

service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
  }
}

16. Expert Round-Up: One Sentence Each

17. Next Steps: Your 24-Hour Action Plan

• Audit current SMTP settings with mxtoolbox.
• Switch to port 587 + STARTTLS if you haven’t.

• Add SPF, DKIM, DMARC TXT records.
• Run mail-tester; aim for 9/10.

• Implement bounce webhook to Slack.
• Warm up new IP if you moved providers.

• Rotate DKIM keys.
• Review queue stats; prune stale accounts.

• Pen-test with swaks & nmap.
• Explore OAuth2 for Gmail users.