How do I determine if an SMTP server "advertises" StartTLS?

Robert Gustafson 606 Reputation points

Visual Basic 2019, .NET 4.6.1, WinForms

I am using SmtpClient to send emails, and the documentation says that if EnableSsl is True when the server doesn't advertise StartTLS, an SmtpException is thrown. I'd like to handle that scenario intelligently.

  1. What is the StatusCode property value for when a send fails for that specific reason? The help info is a little ambiguous on that.
  2. Better yet, is there a way to determine in advance whether a server supports StartTLS and therefore preempt this kind of Smtp exception by setting EnableSsl accordingly?

Please answer both questions ASAP. I'd prefer it in VB.NET, and as simple as possible. (That's what the "S" in SMTP stands for, after all!)

Windows Forms
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