Some suggestions ...
Use a counter cache for messages on Mailbox -- the count of all can be read from the mailbox instance.
Use #size instead of #length. This will use a counter cache for messages on Mailbox if one is present, or will use a SQL count. #length will get all of the messages back, then check the size of the array.
I would be concerned about having multiple mailboxes. If your user model has_many :messages, through: :mailbox you can run these queries more efficiently:
messages = current_user.messages
@today = messages.today.size
@month = messages.week.size
@week = messages.month.size
@all_time = messages.size
With a counter cache, you're only running three queries to get the counts, and thy should be efficient enough.
Place a single index on the columns (user_id, id) on the mailboxes tables, and an index on (mailbox_id, created_at) on the messages table.