SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send emails over the internet. If you need to set up an SMTP hosting server for sending emails, this guide will walk you through everything from selecting a provider to configuring your SMTP server.

Choosing an SMTP Hosting Provider

You can either:

  1. Use a third-party SMTP provider (easier, recommended for businesses).
  2. Set up your own SMTP server (for full control).

Popular Third-Party SMTP Providers

Provider SMTP Server Address Free Tier?
Google SMTP (Gmail) smtp.gmail.com Limited
SendGrid smtp.sendgrid.net 100 emails/day
Mailgun smtp.mailgun.org 5,000 emails/month
Amazon SES Varies by region Pay-as-you-go
Postmark smtp.postmarkapp.com Pay-as-you-go

If you choose a third-party service, you will get SMTP login credentials from them.

Setting Up Your Own SMTP Server (Linux)

To run your own SMTP hosting, you need to install an SMTP server like Postfix, Exim, or Sendmail.

Install and Configure Postfix (Recommended)

Postfix is a popular SMTP server for sending emails.

Install Postfix

sudo apt update && sudo apt install postfix -y # Ubuntu/Debian
sudo yum install postfix -y # CentOS/RHEL

Configure Postfix

During installation, you will be prompted to choose a type of mail configuration. Select "Internet Site" and set:

  • System mail name: yourdomain.com
  • SMTP relay: None (if sending directly)

You can manually edit the configuration:

sudo nano /etc/postfix/main.cf

Add or modify:

myhostname = mail.yourdomain.com
mydomain = yourdomain.com
myorigin = $mydomain
inet_interfaces = all
mynetworks = 127.0.0.0/8
relay_domains =
home_mailbox = Maildir/

Save and exit.

Restart Postfix

sudo systemctl restart postfix
sudo systemctl enable postfix

Test SMTP Server

echo "Test email from my SMTP server" | mail -s "SMTP Test" recipient@example.com

Install and Configure Exim (Alternative)

For CentOS/RHEL users, Exim is a great alternative to Postfix.

Install Exim

sudo yum install exim -y

Enable Exim

sudo systemctl start exim
sudo systemctl enable exim

Configure Exim

Edit the configuration file:

sudo nano /etc/exim/exim.conf

Modify:

domainlist local_domains = @ : yourdomain.com

Save and restart:

sudo systemctl restart exim

SMTP Authentication Setup

To prevent unauthorized usage, enable SMTP authentication.

Install SASL for Authentication

sudo apt install libsasl2-modules sasl2-bin -y # Ubuntu/Debian
sudo yum install cyrus-sasl -y # CentOS/RHEL

Configure SASL for Postfix

Edit:

sudo nano /etc/postfix/sasl/smtpd.conf

Add:

pwcheck_method: saslauthd
mech_list: PLAIN LOGIN

Restart SASL:

sudo systemctl restart saslauthd
sudo systemctl enable saslauthd

Restart Postfix:

sudo systemctl restart postfix

Setting Up SMTP with an Email Client

Once your SMTP server is running, you can configure an email client (e.g., Outlook, Thunderbird, Gmail) to send emails.

SMTP Settings for Email Clients

Setting Value
SMTP Server mail.yourdomain.com
SMTP Port 587 (STARTTLS), 465 (SSL), or 25 (Unencrypted)
Username your_email@yourdomain.com
Password Your SMTP password
Encryption TLS or SSL

Example: Configuring Outlook

  1. Open Outlook.
  2. Go to File > Account Settings > Add Account.
  3. Select Manual setup.
  4. Choose IMAP or POP.
  5. Enter:
    • Incoming mail server: mail.yourdomain.com
    • Outgoing mail server (SMTP): mail.yourdomain.com
    • Username: Your full email address.
    • Password: Your SMTP password.
  6. Click More Settings > Outgoing Server, check "My SMTP server requires authentication".
  7. Click Advanced and set:
    • SMTP Port: 587 (TLS) or 465 (SSL).

Testing SMTP Configuration

Use Telnet

telnet mail.yourdomain.com 25

Type:

EHLO yourdomain.com
MAIL FROM: <you@yourdomain.com>
RCPT TO: <recipient@example.com>
DATA
Subject: Test Email

This is a test email.
.
QUIT

Use Swaks (SMTP Debugging Tool)

sudo apt install swaks -y
swaks --to recipient@example.com --server mail.yourdomain.com --auth LOGIN --auth-user your_email@yourdomain.com --auth-password yourpassword

SMTP Logs & Troubleshooting

Check Postfix Logs

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

Check Exim Logs

sudo tail -f /var/log/exim_mainlog

Common Errors & Fixes

Error Fix
"Relay access denied" Ensure SMTP authentication is enabled.
"Connection timed out" Check if port 25, 465, or 587 is open in the firewall.
"Too many connections" Set SMTP rate limits in /etc/postfix/main.cf.

Secure Your SMTP Server

  1. Enable SPF, DKIM, and DMARC
    • SPF Record (DNS TXT):
      • v=spf1 mx -all
    • DKIM Signing (Postfix):
      • sudo apt install opendkim opendkim-tools -y
    • DMARC Policy (DNS TXT):
      • v=DMARC1; p=none; rua=mailto:postmaster@yourdomain.com
  2. Limit SMTP Relay to Specific IPs

    Add to main.cf:

    smtpd_client_restrictions = permit_mynetworks, reject_unknown_client
  3. Enable SSL/TLS Encryption

    Install Let's Encrypt SSL for secure email transmission:

    sudo apt install certbot -y
    sudo certbot certonly --standalone -d mail.yourdomain.com
  • If you need quick SMTP hosting > Use SendGrid, Mailgun, or Gmail SMTP.
  • If you want full control > Set up Postfix or Exim on your server.
  • Secure your SMTP > Enable authentication, SPF, DKIM, and SSL.