Sending emails in Microsoft Exchange world using an alias for an account has always been a pain. It required working with workarounds such as setting up Shared Mailbox or Distribution Groups and using SendAs permissions. For years admins around the world were asking Microsoft to change this, and finally, in April 2021, they did! It's a new feature of Office 365, and it requires action from Office 365 Administrator.
To enable sending as an alias for all Office 365 users, you need to connect to PowerShell and enable this option. To do so, you must first install the ExchangeOnlineManagement module using the following command
Install-Module -Name ExchangeOnlineManagement -Force
Once installed, connect to your Office 365 tenant using Global Admin credentials.
Verify the status of the Send From Alias option to make sure you know the state of it before applying any change
Get-OrganizationConfig | fl Sendfrom*
Finally, enable Send From Alias functionality for the whole tenant!
Set-OrganizationConfig -SendFromAliasEnabled $true
All that's left is some waiting time as the change propagates thru Exchange Online.