Eliminate the date separation.
The separation by day, earlier this week, earlier this month etc. is EXTREMELY inconvenient and a nuisance. Thank you, I know when emails were received; the date is right at the end of the line. I get an email on, let's say Sunday as a response to a conversation and the **** thing is segregated into the 'last week' category. I thought 'IT WAS ONLY YESTERDAY I WAS HAVING THIS CONVERSATION!' and now my string is broken up. I don't see any reason to segregate my emails into arbitrary divisions of time. I want them concise and all in a row rather than broken up.

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Rik commented
Adding my vote to this. I would much rather just have a clean list of all email headers with nothing separating them.
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Brenda Jamieson commented
I strongly endorse the opinion stated here that the date separators should be made optional. Some people may like them. Clearly, some people absolutely hate them. You can please everyone by including a simple option in Settings.
The problem, from my view, is that they take up valuable screen space and don't give me any information I didn't know already.