Observations :

Recent reported instances of users getting multiple copies of Mails in Outlook Mail Client wherein Old Mails which are already downloaded in outlook re-downloads again from previous dates resulting in duplicate copies.

Cause :

In such cases it was noticed that the mail client is asking for older mails & server in reply is honoring same by allowing those mail to download.

Let us understand more about POP Protocol

• Post Office Protocol(POP) is the primary protocol behind email communication. POP works through a supporting email software client(Microsoft Outlook, Apple Mail, Mozilla Thuderbird, Eudora) that integrates POP for connecting to the remote email server and downloading email messages to the recipient’s computer machine.
• POP is designed to delete mail on the server as soon as the user has downloaded it. However, Mail Clients allow users to specify that mail be saved for some period of time on Remote Mail Server/Webmail popularly known as "Leave a copy on server"
• Each Mail Client maintains a local database of mails it has downloaded till date & accordingly asks for new mails from server & the server gives mails to client which is known as POP action.
• In these cases, it seems the local db of mail client gets corrupted & it does not know which is the latest mail hence asks from server for all mails available on remote mail server/webmail & server responds to this command.

There are various reasons explained for this behavior of mail client like antivirus scanning, same PST for multiple accounts, some of the forums have discussions on same topic - Outlook downloading duplicate mails.

Solution / Workaround :

As this problem is primarily from Mail Client & best resolution of same will be to disable "Leave a copy on server" option However there is a possibility that users would want mails available on Webmail too, In such cases user can try different options as referenced Duplicate E-mails -Prevention

Another workaround at our end is to set POP Date for respective mailbox facing this problem

What is POP DATE - As mentioned above, when the mail client asks for all mails from webmail it becomes multiples in outlook & through POP DATE we can force mail client to not get older day’s mails & only latest date mails by setting today's date. So, even if the mail client will ask for the old data server will only give today's data as POP DATE is set for today.

IT Admin can send the mail to Enterprise Support asking to set pop date