Once you have exceeded the 25 GB email quota for a mailbox, it will stop accepting any new email and any additional email sent to that email address will bounce back to the sender. To solve this problem you’ll need to free up some space by downloading or deleting email from the mail server.
If you are using an email program to download from a POP email account, then just deselect the “leave mail on server” option, so that your email will be permanently downloaded to your computer.
If you are using IMAP or webmail you’ll need to actually delete some of your mail from the mail server (your trash and sent email are a good place to start). When deleting email using Webmail, make sure to select the “purge deleted” option in order to permanently remove your deleted email from the mail server.
If you are ever unsure of how much email storage space is assigned to your website or you want to find out how much of that space you are actually using, you can check your email quota and current usage by logging into webmail.