"Could not find NT account" error from Exchange.55.Mailbox.Create

The problem is usually the time period between when the account is created and when it is available (longer time when high server / network load).

Solution 1 - use the delay command to wait a few seconds for each account to be available

Data.Loop
 Account.Create
 Delay 3000
 Exchange.55.Mailbox.Create
Data.EndLoop

Solution 2 - first create all accounts, then create all mailboxes

Data.Loop   
 Account.Create
Data.EndLoop
Data.Loop   
 Exchange.55.Mailbox.Create
 Exchange.55.Mailbox.SetProperty
 Exchange.55.Mailbox.SetProperty
Data.EndLoop

Solution 2 is quicker since no delays, but notice that there must be a couple of items in the [Data] section for it to work - if there is only one item there will be no time between first account and first mailbox.