I found a solution, thanks for everything

If you overwrite EmailAddresses with the PrimarySMTPAddress, that’s the only address that will be there.

Thanks for the help, but that doesn’t work, the UPN is still registered as proxyaddress. Maybe there is a way to delet it?

Set-MailUser -Identity $UPN -EmailAddress $PrimarySMTPAddress

The Guest Mail User that you create in the script, but I just found out, with:
Update-MgUser -UserId $NewGuestAccount.Id -ShowInAddressList:$True
The UPN appears in the proxyAddress…
Now im trying to find out how i can avoid that and still show the user in the address list.

Do you mean that Mail User object or the User account?

The Guest User is getting 2 entries in the proxy address:
smtp: UPN-Address
and
SMTP: external address

I want to get rid of the UPN in the proxy, but didn’t find a solution yet…

$NewUserProperties = @{
UserType = “Guest”
GivenName = $FirstName
Surname = $LastName
DisplayName = $DisplayName
JobTitle = $JobTitle
Department = $Department
MailNickname = $NickName
Mail = $Contact.PrimarySmtpAddress
UserPrincipalName = $UPN

The UPN comes from $UPN = $NickName + “#EXT#@Office365itpros.com”
The primary SMTP address comes from the migrated contact’s email address.

What issue are you seeing?