Autoconfig: Add extra aliases/identities

[strarsis]

@gronke: Some email mailboxes can have additional aliases/identities/sender emails, e.g. [email protected] may be the primary email address, but it also has [email protected] as additional email address which email is delivered to [email protected] and which can also be used as a sender email address.

Without auto-configuration those additional email addresses have to be set up by the user manually.