No, unfortunately there is not such feature in Mail.dll.
You can either create a service wrapper over Mail.dll and mock it in your unit tests, use test email addresses, or use fake email server run locally (e.g. nDumbster).
We considered this feature, however it was not clear to us, how users would expect every other method (like Connect, ConenctSSL, Login) to work.
Finally we decided not to implement it, as it seems it mixes the responsibilities and there are better approaches (mocks, fake SMTP servers).