User defined and stored email history
Make a way to Save any number of past emails in large blocks (user selectable) anywhere from a single day or up to a year or more into a single file that contains categories of (sender) (recipients) (copies to) (blind copies to) (timestamp) (sent or received) (attachments) and (text body within the email), that can be stored on the users hard drive and searched WITHOUT having to go online. Should be able to specify to what category the search applies. For example, the user might want to search for all emails that came from a certain email address, or were sent to a certain email address, or if the text body of the email contains a certain word or phrase, or was within a certain time range. And user can make combinations of any of those specifications using logical combinations. An example of such a search would be for all emails sent to JoeX@Yahoo.com between March 1, 2019 and June 4, 2020 which contained the text "meet you at".
It is VERY important that the user be able to capture and keep a copy of ALL their emails for any range of dates in files that the user will name for themselves and that these can be kept on the users hard drive and be searched by using software that is resident on their own computer without having to go online. Or, to simply scroll through viewing all the email they sent or received between date X and date Y..
The user might choose to collect all the emails for, say, April, in one file and those for May in another file, or all email for 2019 in another file, etc. etc. Let the user choose and assign how the files are named.