doneWebs is providing email services through a hosting service by One.com. They have developed the following setup guide:
Step 1 - Go to Settings > Accounts & Passwords
Step 2 - Tap Add Account
Step 3 - Tap Other
Step 4 - Tap Add Mail Account
Step 5 - Enter your account details
Step 6 - Select IMAP
Step 7 - Enter your server settings
Step 8 - Save the account
Step 9 - Tap the account you added
Step 10 - Tap your account again
Step 11 - Tap Advanced
Step 12 - Check Mailbox behaviors
Step 13 - Enter IMAP Path Prefix
Step 14 - Done!
In this guide, we show you how to add your email account with IMAP to the Mail app on your iPhone with iOS 11.
Step 1 - Go to Settings > Accounts & Passwords
Open Settings on your iPhone and tap Accounts & Passwords.

Step 2 - Tap Add Account
Tap Add Account.

Step 3 - Tap Other
Select Other as the account type.

Step 4 - Tap Add Mail Account
Tap Add Mail Account.

Step 5 - Enter your account details
Enter the account details for the email you want to add:
Name: this will be visible as the sender.
Email: the address you want to add.
Password: the password you use to log in to Webmail.
Description: a name for the account on your device.
Tap Next to continue.

Step 6 - Select IMAP
Tap IMAP as the account type.

Step 7 - Enter your server settings
Enter the following settings:
Incoming server:
Host Name: imap.one.com
Username: your email account
Password: the one you use for Webmail
Outgoing server:
Host Name: send.one.com
Username: your email account
Password: the one you use for Webmail
Tap Next to continue.

Step 8 - Save the account
Tap Save to create the account. Note that syncing of notes is not supported via IMAP, so we recommend disabling the option.

Step 9 - Tap the account you added
Tap the account you just added. You'll recognise it by the description.

Step 10 - Tap your account again
Tap your account again to open your settings.

Step 11 - Tap Advanced
Tap Advanced at the bottom of your screen.

Step 12 - Check Mailbox behaviours
Check if the Mailbox behaviours are set correctly to make sure that your emails are saved in the right place.
You need to select the following folders on the server:
Drafts mailbox: Drafts
Sent mailbox: Sent
Deleted mailbox: Trash
Archive mailbox: Archive
If no folder, or a different folder, is selected, tap the mailbox and select the correct folder on the server. (The name of the folder is in English.)
Note: If the folder you are looking for is not listed, log in to Webmail and place an email in the folder that is missing. Then try again, it should now show up in the list.

Step 13 - Enter IMAP Path Prefix
Scroll down to the bottom of the screen and enter INBOX as the IMAP path prefix, all in capitals.

Step 14 - Done!
Your email account is now set up correctly on your iPhone.
