If you do not have a Backup MX Server (aka Mail bagging or Backup MX), the following conditions may occur if your primary mail server becomes unavailable for any reason:

• Email will bounce (Returned to Sender)
• Your (inbound) email will cause a backup in the originating mail server's spool
• Service Timeout. Depending on the Retry Attempts by the originating mail server, your mailboxes may never receive their incoming email
• Correspondents may not understand bounce messages. To most users, bounced (undelivered) messages are unreadable, so when they can't send an email, they may not try to resend