By default QMM copies the primary SMTP address of users and groups to the target domain, adding these as secondary addresses. However depending on how the redirector addresses are configured this may not work correctly for groups.
This problem occurs if you have entered the primary SMTP namespace from the source under the properties of the sync job, under "Specify Exchange Options" and "Mailbox redirection settings".
QMM will ignore any SMTP addresses on source groups that match either of these redirection namespaces.
Choose another namespace for redirection. It is recommended that this namespace be dedicated for this purpose and not a "live" namespace.
Page 54 of the Migration Manager for AD User Guide states:
You should analyze your environment for SMTP namespaces and for redirection
implement SMTP address templates that are NOT being used.