This error occurs when you have selected PHP Mail as your mail type for the Mail Provider setting. It indicates that, when attempting to send the email, the mailserver rejected it.

The PHP Mail function uses the server's mail()  function, which is not always reliable. It also doesn't generate a friendly error.

Switching to another Mail Provider in the Mail tab at Configuration > System Settings > General Settings is faster, safer method and would allow you to have more descriptive errors when an issue does occur. These other methods are also usually more reliable when sending emails.

You could also try the following alternative server-side configuration changes that might resolve PHP mail() issues:

• Install a local mail server (for example, Postfix).
• Ensure that your sendmail_path points at the sendmail binary (usually /usr/sbin/sendmail) in your php.ini file.

There are several possible reasons for this error:

• The SMTP settings that you entered for Mail Provider in the Mail tab at Configuration > System Settings > General Settings are incorrect. Check with your system administrator to ensure that you entered the correct SMTP Port, SMTP Host, SMTP Username, SMTP Password, and SMTP SSL Type settings for your mail server.

• You're using an inappropriate SMTP Port setting and have selected SMTP SSL Type. For example, selecting SSL and the port number 25 would cause this error. The most common port assignments for each SSL type are: 
  • None — 25 or 26
  • SSL — 465 or 587
  • TLS — 587
• The mail server is blocking connections from the server that hosts your WHMCS installation.
• The server that hosts your WHMCS installation is blocking outgoing connections to the mail server.

Work with your server administrator or hosting provider to resolve these issues.

This indicates that the mail server is rejecting the SMTP details that you entered for Mail Provider in the Mail tab at Configuration > System Settings > General Settings. Make sure that they are correct.

If you are using Google Mail, this could also be due to the security settings in your email account. For more information, see Suspicious access attempt blocking and Allow applications to access your account.

A Sender Verify Failed error indicates that the sending email address does not exist on the SMTP server.

To confirm this:

1. Log in to cPanel and navigate to Home >> Email >> Email Accounts.

2. Note the email accounts on this page:

3. In WHMCS, go to Configuration > System Settings > General Settings.

4. Verify that Email Address matches one of the email addresses that you saw in cPanel.

Syntax errors are usually due to invalid code in the email template.

The error in Configuration > System Logs provides the following debugging information:

1. The error.
2. The line number of the invalid syntax within the email template.
3. The code surrounding the error.

To resolve the error, go to Configuration > System Settings > Email Templates and click on the email template (in this example, the Client Signup Email template).

Review the code of the email template and correct any errors.

In this example, an erroneous {if} is in the email template. Removing it will correct the syntax.

Click Save Changes.

A Smarty Security Policy Error indicates that the email template contains advanced code that the default email security policy blocks.

Review the code in the email to confirm what it's trying to send at Configuration > System Settings > Email Templates. Then, add an exception to the email security policy to allow the desired code.

For a more detailed explanation of the security policy and instructions for adding an exception, see  Smarty Security Policy.

A Data Not Accepted error occurs when the mail server rejects the email.

• There could be many reasons for this, and you may need to contact your mail server administrator.
• For steps to get more verbose output from your mail server, see Email Sending Issues.

You may see the following common causes for this:

The full error typically displays with a troubleshooting URL:

SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting

This error is typically due to once of the following causes:

• DNS failure — The server cannot resolve the supplied SMTP host.
• Firewall block — A firewall rule is preventing the server connecting to the supplied SMTP host.
• Remote SMTP Disabled — The server configuration may prevent the use of external SMTP servers.
• SSL Certificate Misconfiguration — A certificate on the hostname certificate on the SMTP server may be expired.

To identify the specific cause, follow the advanced troubleshooting steps to enable verbose output of the SMTP connection.