I haven't heard of this and I'm not sure any firm can offer this right now. If your opens declined in July from @gmail.com addresses, then you know your emails are going to Promotions.
A number of newsletter writers have asked readers to move their emails to Priority or to turn off Promotions altogether. You might ask your audience to do this same. Again, only send that to people with @gmail.com addresses. A few firms do use GApps, but I'm not sure how easy it is to filter those people.
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