What is an email alias?
An alias is a way to disguise another Office 365 email address. Everything sent to an alias email address is actually received in the primary email account's inbox. You can create multiple aliases for the same email account.
Aliases can diversify a single email account without having to pay for multiple inboxes. They can also be a way to organize your incoming email with folders or labels for mail sent to a specific alias.
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 firstname.lastname@example.org as her primary email account. She creates the aliases email@example.com and firstname.lastname@example.org. All email sent to jobs@ or events@ goes right to Molly's regular inbox. When she replies to any email, the reply comes from email@example.com.