Outlook doesn't look up smtp-adresses for some NT usernames

I've a problem with the NT Username to SMTP adress lookup in Outlook and Exchange. For most of the users I can write the NT username and Outlook fills in the SMTP adress in the TO: field. For some users (4 of 450) this doesn't work and I have to write the smpt-adress.
Is this a bug? Anyone with a solution or an idea?

Niklas, Sweden
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Are you sure that Mailbox aliasname and nt account are identical?
Outlook uses mailbox aliasname, not nt account

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