If you are migrating by keeping the same domain then yes, they will remain. If you are migrating to a new domain then they will not be migrated by default using 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.