That is what the "Automatic" setting does - you can choose of the regular timed options (15 minutes/30 minutes/hourly) if you want a regular email fetch (regardless of connectivity type and power source).
Note this only applies to pulling email from the server (such as pop or imap email retrieval) apple call this "Fetching". The phone has to make a conscious decision to go look for mail, connect to the server, check for/collect email, and disconnect. This consumes power and data to do all that so tou don;t want to do it frequently on batter and cellular.
Other types of email service that work over https do "Push" email, where there is continuous channel established out to the server over which the server can push email back to the phone as it arrives. When the channel is idle only very small, infrequent 'keep-alive' packets keep the connection established.