It's not possible to configure a space character in the IMAP username field in the email settings of the Lumia phones. The workaround is to ask the email service provider to change the username and remove the space character or replace it with another character. If there's no space character in the corresponding email address, you can try if the IMAP access works OK by using the email address as the IMAP username. In some of the IMAP email services it's possible to use both the user name and the email address as the IMAP username and a separate user name is not necessarily needed.