Sounds like your default address policy is not created or being applied in the correct order.
If these are existing AD accounts, just go into ESM and into Mailbox, new mailbox and it should pull up the list of all users that do not have an exising mailbox select them all next fill in the details.
Gives you very limited options but if your exchange platform is small an not complex might work out better for you.
Kimbie
Import-Csv C:\test.csv | foreach {New-Mailbox -Alias $_.alias -Name $_.Name -UserPrincipalName $_.UPN -Database "Standard Mailbox Database" -OrganizationalUnit Engineers -Password $Password}