From project menu open "References" dialog box. If you have OSSMTP_Plus.tlb properly registered on your machine, you will see an entry for "OstroSoft SMTP Component". Check it and click OK to close dialog box. From this point you should be able to use early binding and look up component properties and methods in Object Browser.
Same applies to VBA projects

Why does new SMTP Component file is called OSSMTP_Plus, not OSSMTP?

To avoid a confusion on computers, having both older and newer versions of SMTP Component installed

Does new SMTP Component support COM?

Yes. Installation package for SMTP Component 7 and later contains typelibrary, providing COM interface to all its objects, methods, properties, events and collections (OSSMTP_Plus.tlb)

Does new SMTP Component support ActiveX?

Do I have to download and run entire installation package for SMTP Component or it's just enough to copy OSSMTP_Plus.dll?

You don't have to run installation, though it's strongly recommended

What's inside the installation package for SMTP Component?

OSSMTP_Plus.dll, OSSMTP_Plus.tlb (self-registering), help file, list of most common component errors, License Agreement and sample projects, covering most popular languages and technologies for Windows

How do I distribute SMTP Component with my application?

Include OSSMTP_Plus.dll in your installation package. Since SMTP Component is not a global assembly, put it in location where your application can easily find it. For example, root application directory on target machine.

Does SMTP Component have any external dependencies?

SMTP Component requires .NET 2.0 or later

Do I need to register OSSMTP_Plus.dll on target machine?

No, OSSMTP_Plus.dll is .NET assembly, not a COM library. Just make sure to put it in location where your application can easily detect it. For example, application installation folder
However, if you are planning to call SMTP Component from COM languages, you do need to register OSSMTP_Plus.tlb