DBMail is another Data exfiltration risk on Azure SQL Managed Instance and it needs to be disabled unless there is a very specific need. Most of the alerting can be now done by log analytics or any other notification service and hence there is now a very limited need to enable DBMail directly on the SQL Instance.
Here is the code to disable DBMail on Azure SQL Managed Instance
sp_configure 'show advanced options', 1;
GO
RECONFIGURE;
GO
sp_configure 'Database Mail XPs', 0;
GO
RECONFIGURE
GO