By default, email message content and attachments will be downloaded when emails are first read from your email account. This will use more storage space and network data but will save time when you want to view an email later.

In the edit email account settings (View) you can open the 'Advanced' section and change the preload settings. 'Preload email content' will download the email message. 'Preload whole email' will download the message and attachments. More

You can also limit the storage space used by each email account. In the edit email account settings (View) open the 'Advanced' section and then tap on the 'Disk Usage' button.
In the 'Disk Usage' screen you can select a maximum storage space usage for this email account. Once the selected storage limit is reached then the message and attachment content email of emails will be removed from disk storage. This information can be downloaded later when you tap to view an email. This setting will not remove emails from your email account or from your inbox. This will only free up storage space used by the message and attachments.

You can tap on the 'Clear' button to clear out any existing disk space used by email message content and attachments. This information will then be loaded later when you need it.