Allow an alternative domain splitter for IMAP authentication

[peter-vanpoucke]

Is your feature request related to a problem? Please describe.
My IMAP provider uses a '.', instead of a '@', as a domain splitter. In order to filter out which domain is allowed, the default character should be configurable.

Describe the solution you'd like
Add a domain splitter to the IMAP authentication.

Describe alternatives you've considered
No filtering on the domain.