Pretty much the same holds for the Open() and OpenMultipart() methods. Actualy these two are more likely to fail than the object creation since at THIS point the module tries to connect to the server.
It's good that Blacksmith tests the result of SendFile(), but the mail is safely sent only if the $sender->Close() said all is OK as well. You can attach several files by several calls to SendFile() and the mail is finished only after you ->Close() it.