No that does not make any sense. When we send emails, we use Apple's built-in email to send the emails. When you send a Report and it opens the Apple email window (and that is already in Apple's hands and not our app anymore), it is the exact same one as you see when you email an Invoice. You do see the email window with the attachment in it, correct? The only thing I can think of is that you have an invalid email address in your Invoice somehow. For example if you have a bad To, Cc or Bcc (has an improperly formatted email, like missing @ or spaces, etc), most email programs will not send an email if one of the email addresses is bad. The other thing you could try to "Compress Archive" in the Invoice Email settings to see if there is something odd with the PDF). For example, we have seen where if you have an invoice with very little info and a Text Heading, some email clients will mange the email. You are best off using a Graphic Header in those cases.
Just as a note, Time Master is at version 6.1 (or greater) on all three platforms now.