Parameter Count under the [PersonDocCustom] section in the MNE configuration has to be set. Also operation “Migrate mailbox data” has to be enabled.

See below details regarding parameter Count below:
[PersonDocCustom] Count= Determines which PersonDoc parameters (see below) the wizard will process in this [PersonDocCustom] section. If a Count parameter is specified with a value > 0, then the wizard will read only PersonDoc parameters whose differentiating digits are less than the Count parameter value. That is, the keyname digits of multiple PersonDoc parameters need not form an uninterrupted numerical sequence, but if they don't the wizard will read only the parameters whose digits are less than the Count value. For example, if: [PersonDocCustom] Count=3 PersonDoc0=FullName PersonDoc2=MailDomain PersonDoc3=FieldName3 PersonDoc5=FieldName5 ... then the wizard will read PersonDoc0 and PersonDoc2, but not PersonDoc3 or PersonDoc5, since neither 3 nor 5 is less than the value of the Count. If unspecified, the Count parameter defaults to 0, in which case the wizard will read only the PersonDoc parameters whose differentiating digits form an uninterrupted numerical sequence beginning at 0 (PersonDoc0).