@IMM
Even if it works immediately after migration, beware that at some time it will change to insist on aliases or mailboxes to send emails.
As @MisterW says, Force9 is already using the Greenby IMAP server which seems to impose the requirement.
However, for about a month after my (Force9) migration it was not a requirement, then it was imposed without warning around the end of January, causing me major problems.
