Configure the server to support Cached Exchange Mode. This downloads a local copy of the mailbox (.ost file) to the user's computer, reducing real-time server queries.

Require MFA for all Outlook connections, especially when users access email outside the corporate network.

Specifies which IP addresses can send mail for your domain.

I can provide a tailored recommendation and setup checklist.

Outlook communicates with mail servers using specific protocols. Your choice of protocol determines how data syncs between the server and the client.