We are migrating a number of domains from one tenancy to another within Office 365.
Tenancy 1 has 18 domains and we cannot migrate all at once.
One major issue we have hit migrating from T1 (Tenancy 1) to T2 (Tenancy 2) is NDR's.
If you migrate a user to T2 and they reply to a mail of a user on T1 they get an NDR.
This appears to be because of the use of X500 (not smtp) to relay mail.
The user can send a new mail OK (they search GAL for user and see a contact for user on T1 which relays mail across)
However any reply to a mail pre-migration produces a NDR as it uses X-500 which will not work in T2.
Is there any way to avoid this issue ?