Update SMTP.ts

[Hiroki111]

sendMessage function doesn't return a string value.

[fzammetti]

Hi! Thanks very much for the contribution! I'd like to merge it, but I wonder if you could make just one minor change before I do: could you please add a comment right above the line mentioning that this is a change from the text in the book and why it's changed?

The concern I have is that if I just accept it outright, the code in the repo will diverge from the code shown in the book, and that will likely lead to confusion for readers (it's a unique concern with code from books versus just code that exists on its own). Adding a comment would provide the reader pertinent information if/when they see the difference.

Thanks again,
Frank