> ## Documentation Index
> Fetch the complete documentation index at: https://documentation.mysmtp.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Common SMTP Errors & Solutions

> Fix frequent mySMTP connection problems including timeouts, authentication failures, TLS errors, and throttling. Step-by-step solutions for each issue.

Here are the most frequent connection problems and what they typically mean:

## 1. Connection Timed Out

**Error Example:** Connection timed out or Could not connect to SMTP host

**Cause:** The client could not reach the SMTP server. This is often due to:

* Network issues
* Firewall rules blocking port 25 or 587
* SMTP server being down or unreachable

**Fix:**

* Check your internet/network connection.
* Ensure the SMTP server address is correct.
* Confirm that the relevant SMTP ports are open.
* Try telnet to test connectivity:

```text theme={null}
telnet smtp.example.com 587
```

## 2. Authentication Failed

**Error Example:** 535 Authentication failed or SMTP Error: Could not authenticate

**Cause:** The server rejected the login credentials.

**Fix:**

* Double-check the username and password.
* Make sure the SMTP server requires authentication (some internal relays do not).
* Ensure the account is active and not locked.

## 3. TLS/SSL Errors

**Error Example:** SSL: CERTIFICATE\_VERIFY\_FAILED or Handshake failed

**Cause:** Issues with establishing a secure connection:

* Outdated SSL/TLS protocols
* Invalid or expired server certificates
* Client doesn't trust the server certificate authority

**Fix:**

* Make sure your client supports modern TLS versions (TLS 1.2+).
* Verify the SMTP server's certificate.
* Update system CA trust store or disable certificate validation (for testing only).

## 4. Blocked by Firewall or ISP

**Error Example:** No specific error, but connection silently fails or times out.

**Cause:** Many ISPs block outbound traffic on port 25 to reduce spam.

**Fix:**

* Try using ports 587 (with STARTTLS).
* Contact your ISP to unblock port 25.
* Use a relay service like mySMTP that supports alternate ports.

## 5. Too Many Connections / Throttling

**Error Example:** 421 Too many concurrent SMTP connections or 421 Service not available

**Cause:** The server is limiting how many connections a single client can make.

**Fix:**

* Implement connection pooling or reuse connections.
* Reduce sending rate.
* Work with your ESP to increase connection limits if needed.

## 6. DNS Issues

**Error Example:** Temporary failure in name resolution

**Cause:** SMTP server hostname can't be resolved.

**Fix:**

* Check that the hostname is correct.
* Ensure your server has working DNS settings.
* Try using the SMTP server's IP address as a test.

## General Troubleshooting Steps

1. **Ping or traceroute** the SMTP server.
2. **Use telnet or openssl** to manually test the connection.
3. **Check logs** from your email client or sending application.
4. **Verify server status** from your email service provider.
5. **Temporarily disable antivirus/firewall** to rule them out.
6. **Try from another network** to isolate the problem.

## Tips for Reliable SMTP Connections

* Use modern encryption (TLS 1.2+).
* Don't hardcode IPs, use hostnames that can be updated.
* Monitor connection rates and timeouts.
* Use connection retries with backoff strategies.
* Keep your client libraries updated.


This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.