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);
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