Currently i don't believe there is any way to block sending of email other than blocking the gmail base. As there is no special sub app for it.
Another thing noticed with version 6.0.0 and content release 417 it is not even blocking gmail login and anything, It might be just configuraiton issue on my part. However if you are testing this and are not able to block sending email blocking gmail base then it might be a bug and might want to open a support case for further investigation. Hope this helps.
As a temporary solution you could block this the old fashioned way. Place an smtp block rule for the google ip ranges. You can do some test sends and capture the data in the logs.
For new feature requests, we are supposed to submit these to our Sales Engineer.
I just tested and was able to block gmail-base and got a application blocked page (decryption required). Blocking SMTP won't help since this is a webinterface to your mailbox.
Without decryption, you just get a connection error, but gmail is unreachable nonetheless.
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