If email sent from your server is not being delivered, the problem usually lies in SMTP configuration, DNS records, or IP reputation. This guide walks through the most common causes and solutions.

Common Causes

Step 1 — Check if SMTP Is Running

# Check if a mail service is installed
sudo systemctl status postfix    # or sendmail, exim

If no mail service is installed:

sudo apt install postfix -y    # Ubuntu/Debian
sudo dnf install postfix -y    # CentOS/AlmaLinux
sudo systemctl enable postfix
sudo systemctl start postfix

Step 2 — Send a Test Email

echo "Test email from server" | mail -s "Test" [email protected]

Check the mail log:

sudo tail -50 /var/log/mail.log      # Ubuntu/Debian
sudo tail -50 /var/log/maillog       # CentOS/AlmaLinux

Look for error messages that explain why delivery failed.

Step 3 — Check DNS Records

SPF Record

SPF tells receiving servers which IPs are authorized to send email for your domain:

dig example.com TXT +short

You should see something like:

"v=spf1 ip4:203.0.113.10 -all"

If missing, add a TXT record to your DNS:

• Type: TXT
• Name: @
• Value: v=spf1 ip4:203.0.113.10 -all

Reverse DNS (PTR Record)

dig -x 203.0.113.10 +short

The result should match your mail hostname (e.g., mail.example.com). Set up reverse DNS through your Data Mammoth dashboard.

DMARC Record

Add a DMARC record for email authentication reporting:

• Type: TXT
• Name: _dmarc
• Value: v=DMARC1; p=quarantine; rua=mailto:[email protected]

Step 4 — Check Port Connectivity

Test if port 25 (SMTP) is open outbound:

telnet smtp.gmail.com 25

If it times out, outgoing SMTP may be blocked. Try ports 587 or 465 instead by configuring your application to use an external SMTP relay.

Using an SMTP Relay

If direct sending is blocked or unreliable, use an external SMTP service:

Configure your application to send through an SMTP relay:

• Host: Your SMTP provider's server
• Port: 587 (TLS) or 465 (SSL)
• Authentication: Your SMTP credentials

Step 5 — Check IP Reputation

Your server's IP may be on a blocklist if it was previously used for spam:

Step 6 — Check Firewall Rules

Ensure outgoing SMTP ports are not blocked:

# UFW
sudo ufw allow out 25/tcp
sudo ufw allow out 587/tcp
sudo ufw allow out 465/tcp
firewalld
sudo firewall-cmd --permanent --add-port=25/tcp
sudo firewall-cmd --permanent --add-port=587/tcp
sudo firewall-cmd --reload

Step 7 — Check Application Configuration

If using PHP:

# Test PHP mail function
php -r "mail('[email protected]', 'Test', 'PHP mail test');"

If using WordPress or another CMS, install an SMTP plugin that sends through an authenticated SMTP server instead of the PHP mail() function.

Prevention

What to Do Next

• Server Networking — IPs, DNS, Reverse DNS — Configure DNS and PTR records.
• DNS Not Resolving — Fix DNS configuration issues.
• Adding Additional IP Addresses — Get a dedicated email IP.
• How to Submit a Support Ticket — Get help with email issues.