If you are porting code from .NET Framework to .NET Core, by all means please use the .NET Standard 2.0 version of Rebex.Imap.dll that was first released with Rebex Secure Mail 2019 R1. It does contain the CheckConnectionState method.
.NET Standard 1.5, introduced in September 2016, was still a rather limited subset of the full .NET API, and the same applies to Rebex binaries targeting that platform. On top of this, these binaries were deprecated at the end of 2019. .NET Core 1.0/1.1 is already an unsupported end-of-life platform, and NET Standard 2.0 binaries are much more suitable for .NET Core 2.0 or higher (and feature essentially the full Rebex Secure Mail API).
Please see our platform support chart for a list of available Rebex binaries and their target platforms.