The modify mailbox function seems to have a problem
with X500 en EUM aliases.

When getting a mailbox the mailadresses array can contain an alias like:

X500:/o=Hosted Exchange/ou=First Administrative Group/cn=Recipients/cn=mymailboxalias
EUM:2200;phone-context=myorganisation.hmc.local

If I then try to modify the mailbox with such an alias, the system throws an error.

The only solution I found so far is with exchange powershell:

$foo = get-mailbox mymailboxalias
$foo.emailaddresses += "X500:/o=Hosted Exchange/ou=First Administrative Group/cn=Recipients/cn=mymailboxalias"
$foo.emailaddresses += "EUM:2200;phone-context=myorganisation.hmc.local"
$foo | set-mailbox

But I wonder If I can do the same thing with MPS webservices?
Does anyone know a solution?