Keeping messages in your INBOX
Do not keep a large number of messages in your INBOX, or use this folder for long term storage. If you wish to keep a large quantity of messages or keep messages for a long time, move them to another folder.
The INBOX is the most volatile mailbox in your e-mail account because the system is frequently updating it with new messages. Although mailbox problems are rare, if problems do occur the most likely victim will be the INBOX. Also, the UWP NetMail web client will perform faster if you have a smaller INBOX folder.
A general rule is to have no more than 500 messages in your INBOX folder, and to keep the size of this folder under 50MB.