Kube currently seems to assume that login username for IMAP and SMTP is the same as the e-mail address -- however, this is not always the case.
Kube should support specifying login username separately from the e-mail address. This could be done via the IMAP and SMTP configuration URLs, if support for a username is provided. This would mean that one could specify them as (for example):
imaps://username@mail.example.com:993 smtps://username@mail.example.com:587