Enable Outlook Online Plus Addressing

Run Powershell as administrator

Set-ExecutionPolicy Unrestricted

Install-Module -Name ExchangeOnlineManagement -RequiredVersion 2.0.3

You will need to respond Yes to 2 prompts

$UserCredential = Get-Credential

make sure the credentials you enter have authority to admin the domain

Connect-ExchangeOnline -Credential $UserCredential -ShowProgress $true

Set-OrganizationConfig -AllowPlusAddressInRecipients $true

May take up to an hour to be fully enabled.

more info here here here