The SMTP error 110 (Connection Timed Out) occurs when your email client or server fails to connect to the SMTP server. This typically indicates network issues, firewall restrictions, incorrect SMTP settings, or the mail server being down.

What Causes SMTP Error 110?

Firewall Blocking SMTP Ports

Some ISPs or hosting providers block SMTP ports 25, 465, or 587 to prevent spam. Your local firewall may be preventing outgoing SMTP connections.

SMTP Server is Down or Not Responding

The SMTP server may be offline, overloaded, or misconfigured.

Incorrect SMTP Settings

Wrong SMTP server address, port, or authentication settings.

DNS Resolution Issues

Your mail client cannot resolve the SMTP hostname (e.g., mail.example.com).

ISP Blocking Outbound Mail

Some ISPs block outgoing SMTP traffic on port 25 to prevent spam.

Rate Limiting by the Mail Server

The mail server may be rejecting connections if too many requests come from your IP.

How to Fix SMTP Error 110 (Connection Timed Out)

Check Your SMTP Server Address

Ensure you are using the correct SMTP hostname and port.

Example SMTP Settings

Provider SMTP Server Address Port (TLS) Port (SSL) Authentication
Gmail smtp.gmail.com 587 465 Required
Outlook smtp.office365.com 587 465 Required
Yahoo smtp.mail.yahoo.com 587 465 Required

If using a custom mail server, verify that mail.yourdomain.com is correct.

Test SMTP Connection Using Telnet

Check if the SMTP server is reachable.

Run this command (Linux/Mac/Windows)

telnet mail.example.com 587

Expected response:

220 mail.example.com ESMTP Postfix

If you get "Connection timed out" or "Could not open connection":

  • The server may be down or blocking SMTP connections.

Check SMTP Server Status

If your SMTP server is self-hosted (Postfix/Exim), restart it:

sudo systemctl restart postfix # For Postfix
sudo systemctl restart exim # For Exim

Check logs:

sudo tail -f /var/log/mail.log # Postfix logs
sudo tail -f /var/log/exim_mainlog # Exim logs

Use Alternative SMTP Ports

Some ISPs block port 25. Try these instead:

  • Port 587 > Recommended (STARTTLS)
  • Port 465 > SSL/TLS
  • Port 2525 > Alternative for some providers

Check Firewall and ISP Restrictions

Run this command to see if a firewall is blocking SMTP:

sudo iptables -L -n | grep 25

If you see DROP or REJECT, unblock SMTP:

sudo iptables -A OUTPUT -p tcp --dport 587 -j ACCEPT
sudo systemctl restart iptables

For Windows Firewall, allow outgoing SMTP:

  1. Open Windows Defender Firewall.
  2. Go to Advanced Settings > Outbound Rules.
  3. Allow port 587 (or 465) for SMTP.

If using cloud hosting:

  • Enable SMTP in security settings.
  • AWS blocks port 25 by default—request unblocking from AWS Support.

Flush DNS Cache (If Using a Domain)

If your SMTP hostname (mail.example.com) is not resolving, clear DNS cache:

Windows

ipconfig /flushdns

Linux/macOS

sudo systemctl restart systemd-resolved

Check Mail Server Logs for Rejections

If your server is rejecting connections, check the logs:

Postfix Logs

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

Exim Logs

sudo tail -f /var/log/exim_mainlog

Look for rate limiting or authentication failures.

Contact Your Hosting Provider

If you have tried everything and SMTP still times out, contact your:

  • Email hosting provider (e.g., Google, Microsoft, your mail host).
  • Web hosting provider (if using cPanel, DirectAdmin, or VPS).
  • ISP (if they block SMTP ports).

Final Fix Summary

Cause Fix
SMTP Server is Down Restart SMTP server or contact hosting provider.
Wrong SMTP Settings Verify SMTP hostname, port, and authentication.
Firewall Blocking SMTP Ports Open port 587 or 465 in firewall settings.
ISP Blocking Port 25 Use port 587 or 465 instead.
DNS Resolution Issues Flush DNS cache and test with ping mail.example.com.