Here is a step-by-step guide to help you configure Plesk mail in iOS.
Go to Settings and tap ‘Mail, Contact, Calendars’.
Select ‘Add account’.
Select ‘Other’ from the list of email providers.
Then tap ‘Add Mail Account’.
Enter your Name, Email, Password and a short description and tap ‘Next’.
Select IMAP or POP and enter your incoming and outgoing mail server details.
Accept the SSL certificate. Tap Continue. This will appear twice, once for your incoming mail server and once for the outgoing one.
Tap ‘Save’ to finish setting up your email.