iOS 18.2, released in early December 2024, brought a significant optional change to the Mail app which will be turned on by default. The release of iPadOS 18.4 in March 2025 brought this feature to the iPad, too. It is similar to a feature that has been available in Gmail for yers. Your Inbox mail is automatically classified into categories named Primary, Transactions, Updates and Promotions. You must select the category you want to view near the top of the Inbox screen. By default, it will be Primary. VoiceOver users will be told which category is selected but won't be told that the category labels are buttons that can be double tapped.
I'm sure that many people will find the new interface helpful but it can be confusing and cause you to overlook or unintentionally delete messages if you don't understand how the new interface works. I think that VoiceOver and low vision users will need to take a careful look at the new interface to ensure they understand it and want to use it. I recommend that you turn it off initially and then when you have time to explore the new interface and discover if it's for you, turn it back on and ensure that you understand how it works and how to use it. Personally, I've never liked this kind of feature and I turned it off immediately.
If you want to turn off the new interface, use the more button, three dots in a circle, at the top right of the Mail Inbox screen. A menu will pop up. To return to the way Mail used to look, select List View from that menu. VoiceOver users can find the more button by swiping right a few times from the top of the Inbox screen or by exploring near the top right corner of the screen. After double tapping the button swipe right once to List View and double tap to select List View. Please note that List View won't be described as a button, check box or whatever; you will just hear List View spoken.
Here's a little more information about the new Mail interface.
Mail in the Primary category behaves much the same as it used to do but the other categories are a little different. By default, the other categories behave like the conversation view in the Messages app. Messages from the same sender are grouped together and only the most recent message is immediately visible. If you choose to delete in this view you will be asked if you want to delete all messages in the group. If you open the group, to view all messages from that sender then you will be able to view and delete individual messages. I'm concerned about the group deletion behaviour because I think it's too easy to delete important messages unintentionally this way. For example, a message with my tickets for an Eventbrite event was grouped together with later marketing messages from Eventbrite and it was one of those later marketing mesages which showed in the list. I could easily have deleted the ticket unintentionally. If you like the classification feature but have concerns about grouping, it's possible to turn off grouping on a category by category basis.
If you prefer not to have your mail sorted automatically into the four categories, this feature may be turned off in the Mail app itself, not in the Settings app. Use the more button at top right of the Inbox list. Its appearance is three dots within a circle. This pops up a menu of choices for Categories, List View, an explanation of how categories work and, for those with Apple Intelligence, the option to highlight messages considered to be priority messages at the top of the Inbox list. If you want to turn off the categories feature, select list view. Your Inbox will return to its original view but messages will be marked with their categorisation with a category icon which is spoken by VoiceOver. VoiceOver users please note that List View won't be described as a button, check box or whatever; you will just hear its name spoken and you should double tap to select it.
Within categories, by default, messages will be grouped together by sender and only the most recent message will be displayed in the initial view. In this view, your only deletion option is to delete the entire group. If you prefer to turn off this grouping behaviour it must be done for each of the three non-primary categories by using the more button at top right of the category's screen and turning off Group by Sender in the pop-up menu that appears.