不使用 SSL 或 TLS 时 MailKit 中的 SSL 或 TLS 连接错误

Question

I am using MailKit to send emails. I don't use SSL or TLS and using port 25. The mail server I am using has port 25 open. What is hapening is that most of the times emails are sent without any problems and sometimes I get the following exception

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An error occurred while attempting to establish an SSL or TLS connection.

The SSL certificate presented by the server is not trusted by the system for one or more of the following reasons:

1. The server is using a self-signed certificate which cannot be verified.
2. The local system is missing a Root or Intermediate certificate needed to verify the server's certificate.
3. The certificate presented by the server is expired or invalid.

See https://github.com/jstedfast/MailKit/blob/master/FAQ.md#InvalidSslCertificate for possible solutions. MailKit.Security.SslHandshakeException: An error occurred while attempting to establish an SSL or TLS connection.

The SSL certificate presented by the server is not trusted by the system for one or more of the following reasons:

1. The server is using a self-signed certificate which cannot be verified.
2. The local system is missing a Root or Intermediate certificate needed to verify the server's certificate.
3. The certificate presented by the server is expired or invalid.

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Here is the code sample I use

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My questions are: 1 - why it tries to use SSL or TLS given that I set not to use it 2 - why it happens randomly

Answer 1

The ConnectAsync() method that takes a bool useSsl argument only specifies whether the initial connection must use SSL (or, as another way of wording it, it specifies whether the port you are connecting to is SSL-wrapped).

As an example of an SSL-wrapped port, for SMTP that would be port 465.

This option does not tell MailKit whether or not to use the STARTTLS command once the plain-text connection is established.

If you want to disable SSL/TLS completely, use the ConnectAsync() method that takes a SecureSocketOptions argument:

await client.ConnectAsync(m_mailServer, 25, SecureSocketOptions.None);