I have the same problem with an invoice not being delivered to a Gmail address that passed a validator test and confirmed it exists.
"Bounced" does not tell what kind of problem it is, which would allow a conclusion on who can do something about it.
There are over 40 different error codes that can come back from a mail server.
https://support.google.com/a/answer/3726730?hl=en
Some of these errors can be addressed ONLY by QB.
Some of these errors can be addressed ONLY by QB and Google/Gmail.
Some of these errors can be addressed by the QB user.
Some of these errors can be addressed by the QB user and its customer.
CC: @CRH2488, @khoyt, and @kmaitl541
