iOS has long supported email aliases for IMAP accounts, but in the most roundabout a difficult way possible, by copy-and-pasting comma separated lists into the account editor. If you still support older iOS devices, take a look at How to set up mail aliases on iPhone, iPad
Modern iOS releases make this much easier.
- Add a normal IMAP account
- Open the Settings app
- Open Mail, Contacts, Calendars
- Edit the account
- Touch the Account line
- Touch the Email line
- Touch Add Another Email…
While it’s not something that made the Keynote at WWDC, it probably should have given how incredibly useful aliases are to email geeks. Unfortunately aliases are not permitted on the Exchange/ActiveSync protocol, and the iPhone still lacks IMAP IDLE support, so you’re left with a choice: Do you want mail pushed to your iOS device, or do you want to send from multiple aliases?