I've inherited an IT department after just 1 year in it, during which the 30-year sole-IT person retired, and his replacement, who hired me, left for a remote position. Shortly before he left, Veeam was implemented as our backup solution, and we moved our on-premise Exchange server to Exchange Online. I realized too late (after our consultant guided me to remove Exchange from both local servers) that aliases on the on-premise Exchange server never migrated to their respective accounts on EO. However, the Veeam service backed up our on-premise Exchange server, so I have access to the files from that system. Can I get information on the aliases so that I can re-implement them in AD and have them populate in EO, preferably without restoring the mail server entirely?
The Exchange servers are store the mailboxes data not AD accounts/aliases. As everyone mentioned, you should restore AD. And it is better to restore both AD and Exchange servers, because it will be more convenient for migrating to Exchange Online from Exchange servers.