That didn't solve the problem, but logging did give me some more clues.
It appears that the google mail server doesn't like the message-id created for email addresses that are not associated with ACS users.
When I added a user with the email address of my google group, then emails would send.
If there is a user with the email address, I get a message-id like the following and google accepts the email.
Apr 21 10:25:03 pfserver postfix/cleanup[21735]: [ID 197553 mail.info] A08FF2AD46: message-id=<mailto:20573.46157836076.18-281+1682699103+59A2ABE1E65FC5B55A5A3DA756E4A8704CD23658@logansw.com>
If there is no user with the email address, the message-id is as follows and it gets rejected.
Apr 21 10:41:45 pfserver postfix/cleanup[21874]: [ID 197553 mail.info] 01E022B6D3: message-id=20573.1682095304.1@logansw.com