The Deleted folder in your email application is not a folder
We've had a few instances recently where customers have asked us why the emails they thought they were keeping safe in the Deleted folder have been disappearing. And the reason is that the Deleted folder is not a folder, despite looking like one. It's more like an email graveyard, or maybe email purgatory, where emails that you've deleted go briefly before they disappear forever. The time period varies between email applications but eventually, all emails that are deleted will be gone for real. Outlook and Gmail keep deleted emails for 30 days after deletion and Firefox is even more efficient with 3-5 days as standard. The only reason they stay there at all is to give you an opportunity to retrieve them if you delete an email accidentally. In summary, if you ever want to see an email again, please don't delete it.
Happily, in a few cases we've had where deleted emails were required after all we were able to retrieve them because the customer had Microsoft 365 and AvePoint Backup which keeps copies of everything ever created in MS365 in perpetuity, including emails.
How to organise important emails
If you do want to keep emails, and everyone does, you need to set up folders to organise them. You can name them accordingly. I've been a bit lazy about this since I don't like too many folders so I've just gone with a overarching 'Old Emails' folder, and one called 'Receipts'. But I've see much more complicated folder arrangements. You can even nest them under each other. Maybe you could organise them by year, or by topic.
Here's how to create folders in Outlook and Gmail:
Outlook - have a look at the right hand menu and chose a folder under which you want the new folder to 'nest'. Inbox or Archive would be a good choice. Right click with your mouse on that folder and select New Folder from the drop down menu.
Gmail - Click on the settings icon (the cog), top right, to access the settings menu. Then select the Labels tap at the top of the menu, scroll down to the Labels section, under which you will find the 'Create New Label' button. Click on that and enter the name of the new folder, and which folder it will nest under.
If you have a different email application you can Google instructions for how to create a new folder if it isn't obvious. The beauty of having folders is that you can remove emails from your inbox once they've been dealt with, at which point your inbox becomes a manageable to-do list, rather than a nightmarish pile of digital clutter hiding the vital administration of your life.