This is what to do to get your iPhone 7 to get connected:

• Go to Settings
• Accounts & Passwords
• Add Account
• Choose ‘Other’
• Add Mail Account
• Input ‘name’, ‘email’, 'Password’ and ‘Description’
• Touch on ‘Next’
• Make sure that IMAP is selected
• Add details for Incoming Mail Server: mail.interdns.co.uk - port 143
• Scroll down and add details in Outgoing Mail Server: smtp.interdns.co.uk
• Touch on Next
• Cannot connect using SSL? - Touch Yes
• Account is verified and added
• Go back into Settings -> Accounts & Passwords
• Touch on the email account
• Touch on Account on next screen
• Touch on Advanced
• Go to Outgoing Mail Server and change port to 25
• Touch on Next
• Account settings are verified
• Done

 This may vary slightly according to iPhone/ios version but hopefully it will be enough of a steer to get it working.