$mail = Import-Csv -Path "D:\mail.csv"
Foreach ($user in $mail)
{
$Alias = $user.MailboxAlias
$pst = $user.pst
## in the File path spicify the location to export the mailbox, it will be easy i f you choose a location where your account has full control right to share drive
New-MailboxExportRequest -Mailbox $Alias -FilePath \\CAS-01\em\$pst
}