Sending emails from dotnet8 web application

For a more robust and secure email solution, modernizing your application and migrating to Google's Gmail API is highly recommended. Utilizing a library like MailKit can simplify this process.

https://developers.google.com/workspace/gmail/api/guides

To effectively troubleshoot the current connection problem, the error message is crucial. If you don't have it, adding logging and exception handling will help us trace the application's behavior. Additionally, the use of SSL port 465 is problematic as SSL is outdated and vulnerable. Try switching to port 587, which uses the modern and secure TLS protocol. Finally, Google's 'less secure apps access' is being deprecated; App Passwords are the recommended alternative for secure access but this might not be effective for an app to app connection.

https://support.google.com/mail/answer/185833?hl=en#zippy=%2Cwhy-you-may-need-an-app-password

gmail support of basic authentication ended 3/14/2025:

https://workspaceupdates.googleblog.com/2023/09/winding-down-google-sync-and-less-secure-apps-support.html

thus the SmtpClient will no longer work with gmail. currently there are no planned updates to SmtpClient (as its legacy and the update would not be transparent). as suggested you must replace with a library like MailKit that supports modern authentication.

Hi Daniel,

You could try the following troubleshooting steps:

1. Switch to Port 587 with STARTTLS

Port 465 uses implicit SSL, which is outdated and can be problematic in some hosting environments. Port 587 with STARTTLS is the modern, secure standard. MailKit example:

using MailKit.Net.Smtp;
using MimeKit;

var message = new MimeMessage();
message.From.Add(new MailboxAddress("Your Name", "******@mydomain.com"));
message.To.Add(new MailboxAddress("Recipient", "recipient@example.com"));
message.Subject = "Test Email";
message.Body = new TextPart("plain") { Text = "Hello from .NET!" };

using var client = new SmtpClient();
client.Connect("smtp.gmail.com", 587, MailKit.Security.SecureSocketOptions.StartTls);
client.Authenticate("******@mydomain.com", "your-app-password");
client.Send(message);
client.Disconnect(true);

2. Enable Detailed Logging

Add exception handling and logging to capture the exact error message when the app fails on IONOS. This will help pinpoint whether it’s a network issue, authentication failure, or TLS handshake problem.

3. Test SMTP Connectivity from IONOS

Run a command like this from your IONOS server:

openssl s_client -connect smtp.gmail.com:587 -starttls smtp

If this fails, it’s likely a network or firewall issue despite IONOS’s assurances.

4. Google Workspace Configuration

• Ensure the App Password is valid and not revoked.
• Check if SMTP relay is enabled in your Google Admin Console.
• Consider whitelisting your IONOS server IP under:
  • Apps > Google Workspace > Gmail > Advanced settings > SMTP relay service

5. DNS Records

SPF, DKIM, and DMARC are great for email deliverability but don’t affect SMTP authentication. Still, make sure your SPF record includes:

v=spf1 include:_spf.google.com ~all

Also as suggested my AgaveJoe, using the Gmail API is a more secure and scalable approach, especially for production apps.

Benefits:

• Uses OAuth2, eliminating the need for app passwords.
• Avoids SMTP limitations and firewall issues.
• Provides access to advanced Gmail features (labels, threads, attachments).

Getting Started:

• Register your app in Google Cloud Console.
• Use libraries like Google.Apis.Gmail.v1 or MailKit with OAuth2.
• Follow the Gmail API Quickstart for .NET