You can verify an OmniFocus Mail Drop address. I have gmail filters configured to forward certain emails to Mail Drop.
When you first try to use a Mail Drop address in a filter, it will send an email to your Mail Drop address with a link in it. You should be able to open that link in a web browser and that will verify the Mail Drop address.
When you first try to use a Mail Drop address in a filter, it will send an email to your Mail Drop address with a link in it. You should be able to open that link in a web browser and that will verify the Mail Drop address.
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