In most cases this error indicates that the SMTP server requires TLS 1.2 or higher, while
AspEmail attempts to communicate with it via TLS 1.0.
AspEmail and AspEmail.NET offer support for TLS 1.2 as of Version 5.5. If you are using an earlier version, please upgrade.
If you are already using version 5.5+, make sure your script does not use specify the TLS version to be "1.0" via the property TLSVersion.
Also, some SMTP servers which require TLS 1.2 also require that the client application support RSA/SHA256-based signatures, and throw the above error otherwise.
AspEmail and AspEmail.NET offer support for RSA/SHA256-based signatures as of Versions 188.8.131.52 and 184.108.40.20689, respectively.