Resolving Syntactically invalid EHLO argument SmtpException in C#

Learn how to resolve the 'Syntactically invalid EHLO argument' SmtpException in C#. This guide provides solutions for fixing EHLO command issues in scalable cloud environments like Azure, ensuring reliable email delivery in your applications.

When sending emails from your .NET app using a native SmtpClient, you may encounter the following error:

Syntax error in parameters or arguments. The server response was: Syntactically invalid EHLO argument(s)

with the below stack trace:

System.Net.Mail.SmtpException: Syntax error in parameters or arguments. The server response was: Syntactically invalid EHLO argument(s)
at System.Net.Mail.EHelloCommand.CheckResponse(LineInfo[] lines)
at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port)
at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port)
at System.Net.Mail.SmtpClient.Send(MailMessage message)

The issue is be related to the EHLO (Extended Hello) command, which is part of the SMTP protocol.

When the EHLO command is sent to the SMTP server, it typically includes the machine's hostname.

The server can respond with an error if the hostname contains invalid characters or is too long.

Here is invalid hostname pattern that was causing error in my case:

Machine name: myproject-dev_7e6f9fdf

What's the maximum hostname length when sending emails?

The maximum length for a hostname is defined by various standards and practices.

According to the Domain Name System (DNS) standards, a fully qualified domain name (FQDN) has a maximum length of 253 characters.

However, for individual labels (the segments between dots), the length is limited to 63 characters.

Adhering to these constraints is particularly important for SMTP and the EHLO command to ensure compatibility and avoid errors.

Solution: Using C# Reflection to Set ClientDomain explicitly

This approach is not typically recommended due to its complexity and maintenance overhead, but it is a way to address your issue directly.

Below is a sample code you can use:

private SmtpClient CreateSmtpClient()
	var smtpClient = new SmtpClient(SolutionConstants.SmtpSettings.AppEmailSMTPHost, SolutionConstants.SmtpSettings.AppEmailSMTPPort)
		Credentials = new NetworkCredential(
		Timeout = 5 * 1000,
		EnableSsl = SolutionConstants.SmtpSettings.EnableSsl

	// Use reflection to set the ClientDomain field
	var clientDomainField = typeof(SmtpClient).GetField("_clientDomain", BindingFlags.Instance | BindingFlags.NonPublic);
	if (clientDomainField != null)
		clientDomainField.SetValue(smtpClient, "");

	return smtpClient;

You may ask why I am using reflection here.

The reason is obvious: SmtpClient does not expose public "ClientDomain" property.

Problem with Sending Emails in Scalable Cloud Environments

The issue you're encountering is common in scalable cloud environments, such as Azure, due to the dynamic nature of these platforms.

In cloud environments, especially those that automatically scale, instances of your application can be created and destroyed based on demand.

Each instance may have a different hostname, leading to inconsistencies when the EHLO command is sent to the SMTP server.

Final words

In a scalable cloud environment, the frequent creation and destruction of instances mean that the hostname can change often, leading to repeated occurrences of this issue.

By explicitly setting a valid and consistent EHLO domain name, as we did with the ClientDomain workaround, you ensure that the EHLO command remains valid regardless of the underlying instance's hostname.

This approach helps maintain reliable email functionality in a dynamically scaling cloud environment.

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