Can't use UPN as username when connecting to Exchange 2010 SMTP

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http://serverfault.com – I currently have an issue where users can't access SMTP (via Outlook etc) on the Exchange server using the logon format 'user@domain.com', but it works with DOMAIN\user Using the UPN works fine for IMAP, OWA, ActiveSync, just not SMTP. Any suggestions much appreciated! (HowTos)