LegacyExchangeDN after Exchange Online Migration

Fun fact I wasn’t aware of, but learnt today thanks to a colleague.

Say you have an on-prem account with Exchange mailbox. It has a LegacyExchangeDN attribute. When you do an MRS migration of it to Exchange Online this LegacyExchangeDN attribute changes.


/o=xxx/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=aaa-bbb ccc


/o=xxx/ou=External (FYDIBOHF25SPDLT)/cn=Recipients/cn=ddd

Important to know this in case you are also moving the account across forests at the same time (coz… why not!). Must take care to capture this new LegacyExchangeDN to the new account as a proxyAddress.