Se você estiver migrando mantendo o mesmo domínio, então sim, eles permanecerão. Se você estiver migrando para um novo domínio, eles não serão migrados por padrão usando o ADMT.
ADMT and Exchange Attributes: By default ADMT doesn’t migrate Exchange attributes including “mail”, “proxyAddresses”, anything started by msexch. The reason behind that because when ADMT transfers Exchange attributes (e.g. homeMDB, homeMTA, showInAddressBook, msExch*) that will make the target user looks like a legacy mailbox in the target domain. This leaves the target account in an invalid state (e.g. homeMDB still points to the old forest) which is unexpected for the PrepareMoveRequest.ps1 script. To prevent this, Exchange attributes are excluded from ADMT.
Referência: link