Mail.dll is a .NET assembly.
As far as I know Visual FoxPro does support using .NET assemblies through COM Interop.
Most likely the easiest way is to create a .net project (referencing Mail.dll) wrap it using COM interop and use that from Visual FoxPro.
Here are the general steps to use .NET assemblies in Visual FoxPro:
Create a .NET Assembly:
Develop your .NET assembly using a .NET language like C# or VB.NET. This assembly will contain the classes and methods you want to use in your Visual FoxPro application. It'll also reference Mail.dll.
Compile the .NET Assembly:
Compile the .NET assembly using a .NET compiler, which will generate a DLL file containing your .NET code.
Register the .NET Assembly:
You might need to register the .NET assembly in the Global Assembly Cache (GAC) if it's meant to be used globally.
Use the .NET Assembly in Visual FoxPro:
In your Visual FoxPro code, you'll use COM Interop to instantiate and interact with classes from the .NET assembly.
We'll support you during the development, however have in mind we are .NET experts, and not really FoxPro experts.