Possibly this might be a better script to base things on given that things have changed a lot since the article was written: https://office365itpros.com/2022/10/18/update-user-email-upns/

For shared mailboxes, I would use the mailbox alias for the aaddress.

$NewAddress = $null

#If -MakePrimary is used the new address is made the primary SMTP address.
#Otherwise it is only added as a secondary email address.
if ($MakePrimary)
{
$NewAddress = “SMTP:” + (Get-User $Mailbox).FirstName + “.” + (Get-User $Mailbox).LastName + “@$Domain”
}
else
{
$NewAddress = “smtp:” + (Get-User $Mailbox).FirstName + “.” + (Get-User $Mailbox).LastName + “@$Domain”
}

Never Mind.

It had to do with authenticating to Exchange Online using Powershell 5.1. I installed Powershell 7 and it ran flawlessly.

Thanks Paul!

Get-AcceptedDomain : The term ‘Get-AcceptedDomain’ is not recognized as the name of a cmdlet

This is an M365 system only, no on-prem AD or directory sync.

Hi,
I always get this Answer. Whats wrong:

Cannot process argument transformation on parameter ‘Identity’. Cannot convert the “Thomas Barbara” value of type
“Deserialized.Microsoft.Exchange.Data.Directory.Management.Mailbox” to type
“Microsoft.Exchange.Configuration.Tasks.UserIdParameter”.
+ CategoryInfo : InvalidData: (:) [Get-User], ParameterBindin…mationException
+ FullyQualifiedErrorId : ParameterArgumentTransformationError,Get-User
+ PSComputerName : outlook.office365.com

I modified the script on the below link to be using Distribution Groups. However if you have AD Sync then it will use that instead.

https://github.com/dbeato/scripts/blob/master/Office%20365/ADD-SMTPAddress-DG.ps1