How to Clean Up my Mailbox?
The easiest way to clean up your mailbox is through Webmail.
You can access Webmail directly at http://yourdomain.com/webmail with your email account, where yourdomain.com is your website hosted with SiteGround.
The next step is to delete the messages that you don't need and empty your Junk/Trash folders in order to save space.
In case you are using an email client such as Thunderbird, Outlook, or MacMail you can do the same via your email client.