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That is because you are using IMAP on both devices. If you switch to POP messages will be kept, but you will have sync issues. Example: when you delete a message from a phone, it won't delete from your desktop running Outlook 2016 therefore, you won't have the same updated sync status on the devices. Is that what you want to do?
You will need to delete and re-add your account on Outlook 2016 as a POP account.
Would you like me to connect with you remotely to do this? Thanks
see steps here http://in5stepstutorials.com/ms-outlook/manually-add-email-accounts-in-outlook-2016-2013-2010.php
And here: https://support.office.com/en-us/article/Add-an-email-account-in-Outlook-for-Windows-70c2b230-9d23-4c41-98c5-a639e68e06dc
Comcast Settings for POP