I have versions 2.15 of Mailkit and MimeKit. Mimekit is dependent on System.Buffers 4.5.1. When running the code through VSCode there is no problem. But in Powershell Desktop it does not work because it is looking for System.Buffers 4.0.2.0 (which is not even available on nuget). One odd thing on top of this is that it still works in VSCode when I comment out the line to add the System.Buffers.dll, maybe this is a clue.
Add-Type -Path "C:\Program Files\PackageManagement\Nuget\Packages\system.buffers\4.5.1\lib\netstandard2.0\System.Buffers.dll"
Add-Type -Path "C:\Program Files\PackageManagement\NuGet\Packages\mimekit\2.15.0\lib\netstandard2.0\MimeKit.dll"
Add-Type -Path "C:\Program Files\PackageManagement\NuGet\Packages\mailkit\2.15.0\lib\netstandard2.0\MailKit.dll"
Exception calling "Send" with "1" argument(s): "Could not load file or assembly 'System.Buffers, Version=4.0.2.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The system cannot find the file specified."```