Allow configuring lookups for users and their mail addresses from an
LDAP directory. The LDAP username will be used as an accountname as
opposed to the email address used as the `loginName` for declarative
accounts. Mailbox for LDAP users will be stored below
`/var/vmail/ldap/<account>`.
Configuring domains is out of scope, since domains require further
configuration within the NixOS mailserver construct to set up all
related services accordingly.
Aliases can already be configured using `mailserver.forwards` but could
be supported using LDAP at a later point.
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