A complete vendor solution is available. Either the vendor has issued an official patch, or an upgrade is available.
Unpatched or Unsupported Microsoft Exchange Servers Can Expect an SMTP 450 Error
On Monday, March 24th, 2023, Microsoft announced that it is enabling a transport-based enforcement system in Exchange Online serving three functions: reporting, throttling, and blocking.
Microsoft Outlook Privilege Elevation Vulnerability (CVE-2023-23397)
CISA added a zero-day vulnerability affecting Microsoft Outlook that is actively exploited in the wild. Microsoft has released a patch for the vulnerability as part of their March 2023 Patch Tuesday.