Outlook Performance When Enabling A Retention Policy In Exchange Online

Hi @Mitch King

According to this link: Understanding of Managed Folder Assistant with retention policies
(It is about Exchange On-premises but should also apply to Exchange Online)

After the MFA has applied a new policy to a mailbox, the next time the user connects to the mailbox with a client that supports retention policies, user will see that retention tags are shown on items and the retention policy options are visible.
Another important point that you should understand is that if you apply a retention policy that contains a default policy tag, the MFA stamps the default tag on every item in the mailbox.
This action forces Outlook to download the complete contents of the mailbox the next time the client connects and synchronizes with Exchange. Such a massive synchronization has the poten­tial to flood a network and keep clients fully occupied for a long time.
Including a default archive tag in a policy does not have the same effect because the MFA does not stamp every item with this tag.

I suppose it may cause Outlook to resync with Exchange Online server.
And if you have a large OST file, it would take more time.

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