What is an email alias?
An alias is a way to disguise another Microsoft 365 email address. Everything sent to an alias email address is actually received in the primary email account's inbox.
Aliases can diversify a single email account without having to pay for multiple inboxes. You can create up to 400 aliases for the same email account.
However, you can't send from an alias address. Any reply will show your primary email address as the sender. (Check out distribution lists for that functionality.)
For example, Molly has email@example.com as her primary email account. She creates the aliases firstname.lastname@example.org and email@example.com. All email sent to jobs@ or events@ goes right to Molly's regular inbox. When she replies to any email, the reply comes from firstname.lastname@example.org.