Solving the NET Core Mailkit Issue: How to Send Multiple Emails Without Errors

Описание: Discover how to fix the MailKit error in a .NET Core 3.1 project when sending multiple emails. Learn the proper way to reuse connections effectively!
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This video is based on the question https://stackoverflow.com/q/64382865/ asked by the user 'Richard Nguyen' ( https://stackoverflow.com/u/9902154/ ) and on the answer https://stackoverflow.com/a/64389731/ provided by the user 'jstedfast' ( https://stackoverflow.com/u/87117/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the NET Core Mailkit Issue: How to Send Multiple Emails Without Errors

When working on a .NET Core project, many developers encounter issues when using MailKit to send multiple emails. In this guide, we’ll explore a common problem faced by developers and provide an effective solution that ensures smooth email delivery.

The Problem: Challenges with Sending Multiple Emails

In a .NET Core 3.1 application using MailKit, you might find that attempting to send multiple emails can lead to frustrating errors. The general structure of the function could look something like this:

[[See Video to Reveal this Text or Code Snippet]]

The error arises when you attempt to connect to the SMTP server repeatedly in quick succession. For instance, you might attempt to send two emails using the same credentials, but unfortunately, you receive an error after a certain interval. The error message typically reads:

[[See Video to Reveal this Text or Code Snippet]]

Common Causes of the Error

This error could be attributed to several issues:

The SSL certificate from the server is not trusted.

You might be using a self-signed certificate which cannot be verified.

Missing Root or Intermediate certificates on the client side.

The server's certificate is expired or invalid.

Furthermore, switching the SecureSocketOptions from StartTls to Auto might provide temporary relief, but it will not resolve the underlying connection problem when sending multiple emails in succession.

The Solution: Reusing the SmtpClient Connection

The key to solving this issue lies in reusing the same SmtpClient connection instead of connecting and disconnecting for each email you wish to send. This approach not only makes your code cleaner but significantly improves performance.

Steps to Implement the Solution

Modify the SendEmail Method: Change your original method to accept an SmtpClient as a parameter.

Reuse the Connection for Sending Emails: Establish the SMTP connection once and then use it to send multiple emails.

Here’s how the code should look:

[[See Video to Reveal this Text or Code Snippet]]

Implementation Breakdown:

Single Connection: The using var smtp = new SmtpClient(); line establishes a single connection that lasts for the duration of the SendEmail() function.

Email Sending: Each call to SendEmail(smtp, ...) sends an email using the same connection.

Secure Disconnection: Finally, smtp.Disconnect(true); gracefully closes the connection.

Conclusion

By adopting this method, you ensure that the connection established to the SMTP server isn't repeatedly opened and closed, which is often the root cause of connection errors when sending multiple emails. Using MailKit efficiently can significantly enhance your application's email functionality.

If you encounter this error in your .NET Core application, remember to reuse your SmtpClient connection—this simple change can save you time and frustration!