If you’re using Outlook or Microsoft Mail for email, and are not using a smartphone, check that you are not “leaving copy of messages on server” in your Advanced/More Settings. If you are using a smartphone, then check that you are not “leaving copy of messages on server” for an extended number of days – 3 is generally long enough for you to retrieve messags on both your smartphone and your PC.
If you’re using a smartphone to send/receive email, open a browser window and go to http://www.yourdomain/webmail/, where “yourdomain” is you domain name. Log in to your mailbox with your full email address and mailbox password. You’ll be presented with a landing page like this. Then choose one of the 3 browser-based email programs listed to review and delete old messages in your Inbox and Sent folders. Before exiting, be sure to empty/purge your Trash folder or the messages you sent to Trash will remain on the server.