Recently I have implemented same. Please follow below step:
1. Create Table and field would be:
MailMessage (Table)
MessageId (PK)
SenderUserId
RecieverUserId
Subject
Message
IsRead (To view read/unread mail)
DeletedBySender
DeletedByReciever
ArchiveBySender
ArchiveByReciever
MessageOn (DateTime)
2. For Inbox: Your query will fetch info based on RecieverUserId
3. For SendItem: Your query will fetch info based on SenderUserId
Hope so will help you!
Thanks