Plugin which allows you to set up custom username (login) by email address.

Some mail servers use arbitrary usernames for logging in to the email service.
This plugin replaces pre-@ email address with the predefined username for logging in.

Usage:   <user email>:<username>
Example: bob.jackson@mydomain.com:mydomain3
By doing the above, users can log in with "bob.jackson@mydomain.com" instead of "mydomain3@mydomain.com"