Ran an email hosting service and used zimbra. Email migration (through acquiring failed shops that were tech clueless) and replication is nontrivial because of

Ran an email hosting service and used zimbra. Email migration (through acquiring failed shops that were tech clueless) and replication is nontrivial because of edge cases. Backup, backup, backup and test a zillion times before doing it for real. Rolling anything your own or using OSS as-is rarely scales. Neither are commercial products.