What do \I have to do in my web mail to automatically delete emails after they have been delivered to my in box?
This is actually a setting that you would make in your email program or app. The default in older Microsoft programs was to delete from the server after downloading to the program's inbox. But in Mac Mail and the newer Microsoft programs, the default is to leave mails on the server.
For example, here's how to change that in different versions of Microsoft Outlook - Delete Junk Email from mail server too when deleting from Outlook.
If you use a different program, let us know which one and we should be able to find those settings for you.
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