Hmm, seems to work on the first user but every attempt after fails:
C:\DST2007>SetSendAsPermission -a Domain\besadmin -u
Set the Send As Permission in Active Directory tool Version 4.1.2.6
Copyright (c) Research In Motion, Ltd. 2000-2007. All rights reserved.
Modification date: Feb 8 2007
[20000] (17:17:47.875):{0x01D4} SMTP address:
[20000] (17:17:50.422):{0x01D4} FAIL
[10000] (17:17:50.422):{0x01D4} SetSendAsPermission(): Unable to update the NTSe
curityDescriptor