2. At the bottom, go to Exchange admin
3. Click on Mail Flow down the left hand side and select accepted domains across the top to get to here (domain names hidden to protect the innocent!)
Although it wasn’t stated, I suspect the propogation issue is one related to mail enabled public folders and not mailbox or shared mailbox aliases, but I have no evidence for htat.
But, my recommendation? Unless there is a good reason for not doing so, setup up your domains as internal relay before, during and for a few weeks after your cutover migration.