l (Lookup)

If you grant a user the Lookup access right, that user will be able to see this mailbox when it asks the Server to list all mailboxes in your Account.

r (Read/Select)

If you grant a user the Read access right, that user will be able to open (select) this mailbox and see (read) the messages in this mailbox.

s (Seen)

If you grant a user the Seen access right, that user will be able to mark messages as read (seen). Usually a message is automatically marked as seen when a user reads it. But if this access right is not granted to a user reading the mailbox, the mailbox message "seen" status will not be changed.

w (Write/Flags)

If you grant a user the Write access right, that user will be able to set message flags: i.e. to mark messages as answered or "flagged", and to reset the message flags.

d (Delete)

If you grant a user the Delete access right, that user will be able to mark messages as deleted and to compress the mailbox, removing all its messages marked as deleted.

i (Insert)

If you grant a user the Insert access right, that user will be able to append messages to this mailbox and to copy messages from other mailboxes into this one.

p (Post)

This access right is not used by modern mailers.

c (Create)

If you grant a user the Create access right, that user will be able to create new submailboxes "inside" this mailbox.

a (Administer)

If you grant a user the Administer access right, that user will be able to modify the ACL for this mailbox.