Hi, and thanks for buying the book.
That error can be due to a couple of reasons. For example, if the strTo variable contains an illogical recipient name (such as, in a cell, a name has a space or disallowed character), VBA would raise an error. It might also be due to a server issue or a default IT setting that does not allow, or expect that, Outlook will be required to send the email. Hard to say without knowing the background of the environment you're working with.