When securing xDB data with Azure Key Vault, sending lots of emails in one dispatch might fail with an unhandled exception due to the excessive load on Azure Vault. An error similar to the following can be found in the log records:
Microsoft.Azure.KeyVault.Models.KeyVaultErrorException: Request was not processed because too many requests were received. Reason: VaultOperationLimitReached
Upgrade your solution to Sitecore XP 9.3 or later.
To resolve the issue, for Sitecore XP 9.1.1, download and install the hotfix: SC Hotfix 410266-1 Sitecore.Xdb.Collection.Data.SqlServer 2.0.1.zip
Be aware that the hotfix was built for a specific Sitecore XP version and must not be installed on other Sitecore XP versions or in combination with other hotfixes. In case any other hotfixes have already been installed on a certain Sitecore XP instance, send a request for a compatibility check to Sitecore Support.
Note that the ZIP file contents must be extracted to locate installation instructions and related files inside. The hotfixes must be installed on a CM instance and then synced with other instances using standard development practices.