In QMM 8.6/8.6.1, if a migrated account has an existing mailbox prior to the Mail Target Agent logging in, this mailbox may be deleted automatically by the Mail Target Agent.
The Mail Target Agent has a configuration parameter in the config.ini file called "ForbidDelMailbox". This parameter tells the agent to automatically delete target mailboxes if the value is 0, and to not delete target mailboxes if the value is 1.
In previous versions of QMM, this value defaulted to 1 to protect mailboxes from accidental deletion. However, the default in QMM 8.6.1 is now 0, meaning any existing mailboxes associated with migrated accounts will be deleted.
The workaround to this issue is to manually edit the Mail Target Agent's config.ini file so ForbidDelMailbox=1
Alternatively you can use the GUI to change this value and make the setting persistent: