if(-not(Get-PSSession)) {

    . 'D:\Program Files\Microsoft\Exchange Server\V15\bin\RemoteExchange.ps1'
    Connect-ExchangeServer -auto -ClientApplication:ManagementShell

}

$Mailboxes = Get-Content .\shared.txt | Get-Mailbox
$ExOSuffix = "<tenant>.mail.onmicrosoft.com"

foreach($m in $Mailboxes) {

    $NewAddress, $PrimaryAddress, $PrimaryAddressName = $null
    $PrimaryAddress = $m.EmailAddresses | Where-Object {$_.IsPrimaryAddress -and $_.PrefixString -eq "SMTP"}
    $PrimaryAddressName = $PrimaryAddress.AddressString -replace("@.*","")
    $NewAddress = $PrimaryAddressName,$ExOSuffix -join("@")
    $m.EmailAddresses.Add("smtp:$NewAddress")
    Set-Mailbox -Identity $m.Identity -EmailAddresses $m.EmailAddresses

}