Your Nuget package for Aspose.Email 20.3.0 does not properly work for .net 4 profile when building a multi target lib which is referencing the package.
Instead of referencing the net40 libs, the netstandard2.0 libs are referenced.
The reason for that ism that the nuspec is only has dependencies defined for netstandard.
<dependencies>
<group targetFramework=".NETStandard2.0">
<dependency id="System.Text.Encoding.CodePages" version="4.4.0" />
</group>
</dependencies>
Its should be like with Aspose.HTML package to work correctly.
<dependencies>
<group targetFramework=".NETFramework2.0" />
<group targetFramework=".NETFramework3.5" />
<group targetFramework=".NETFramework3.5-Client" />
<group targetFramework=".NETFramework4.0" />
<group targetFramework=".NETFramework4.0-Client" />
<group targetFramework=".NETStandard2.0">
<dependency id="System.Drawing.Common" version="4.5.0" />
<dependency id="System.Security.Permissions" version="4.5.0" />
<dependency id="System.Text.Encoding.CodePages" version="4.5.0" />
</group>
</dependencies>
This package could be fixed quickly without a minor release.