For those that need more available memory on their iPhone, try deleting the emails in your Trash folder. In email go to the email account and then go into the Trash folder. Tap edit and you'll get an option to "Delete All". Select it and confirm the deletion. If you have multiple email accounts, you'll need to perform this for each account.

To keep the size of your email Trash from building up, you can edit your settings so that it is employed more frequently. Go to "Settings" > "Mail, Contacts, Calendars" > "Accounts (pick the actual account)" > "Advanced". Then in the "Deleted Messages" section update the "Remove" setting to be something more frequent. Again, this needs to be done once for each account.