server emails :
this is the first place where you get an email.
What happens if you delete from server?
it will be gone permanently and will not be downloaded to computer or phone if you delete first from the server.
If you delete after you downloaded emails on outlook, then the emails deletion process on server will not impact your outlook.
outlook emails :
the same email will be downloaded to your computer on outlook.
outlook can work independently from your webmail server. it can download the emails and even if you delete from the server, it will not be deleted from outlook automatically,
unless we attempt to delete from outlook.
In case if you want to delete automatically from server when you delete from outlook, here shows some options.
NOTE: Remove from server when deleted from Deleted Items.
Mark this as an important point.
I am sure you might be checking emails only on outlook irresepctive of your mail server – so called as webmail.
The general practice you have to follow here is you need to empty the deleted items on outlook.
right click on deleted items folder and click on empty the folder.
so that your emails will not be re-downloaded again on outlook from the mail server and your emails will be deleted from the server as well.
let me show you an example.
it is on the deleted item now. but you need to empty this one.
it means delete from the deleted items folders.
so that it will be permanently deleted.
the same email will be downloaded to your phone on mail app.
phone will be last priority, what ever it shows on webmail will appear on phone.
it means if you have deleted the emails permanently from outlook, then they will also be deleted from your web mail and it will automatically be deleted from your phone.