Gmail – Search for Old Emails That Are Not Starred

Clearly there is no daily quota - unless my experience is untypical. It can be months before even a single one of my Gmail items is sent to Spam. I have been careful about disclosure of my Gmail address (I use an address at a different service provider when obliged to provide such a means of contact to those I do not fully trust. I have even set up a temporary account at a third service provider for a contact I did not trust at all.)

As you mention, the general public does not know all the details of how Gmail decides what to treat as spam. What we do know, or can predict with reasonable certainty, is that a lot of the factors taken into consideration are to do with the message itself or the sender - not the recipient. For example blacklists of sources, content analysis (including Subject, other 'header' details, multiple addressees), frequency of submissions and spam reports.

A recipient can do little to affect this other than to report as spam (no use to you when you want less aggressive treatment) or, probably, to delete without opening the mail.

So I suggest you either "Mark as read" before deletion, or you open emails before deleting those from people you do not want to be treated as potential sources of spam. Moving them (even if unread) to a folder (other than Spam!) prior to deletion might help.

The filters search against all messages, and then returns entire conversations (assuming the user has turned on Conversation View in the Settings). If any message within a conversation is returned by the query, then the entire conversation is returned, even if the conversation as a whole is not starred.

Therefore using the -in:starred will only work if ALL the messages in the thread are starred.

What is misleading about the Gmail UI is that starring the conversation, does not star all the containing messages, and therefor won't remove it from the filtered inbox when using the -in:starred search option, despite appearing to be starred in the main inbox view.

The syntax of the filter is not important, Gmail accepts both -in:starred and -is:starred with or without the AND keyword.

Of course, if you do not use Gmail in Conversation View (Settings → General → Conversation View), then the filter will work as expected.