Allow an alternative domain splitter for IMAP authentication

**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.
