When an email account exceeds its quota, several things happen. First, the account owner will no longer be able to receive any incoming messages. Senders will receive a mail delivery failure notification stating that the mailbox quota has been exceeded. This means that any new messages sent to the account will not be delivered.
Additionally, the account owner will still be able to send outgoing messages, but they will receive an error message stating that the message could not be saved. Normally, when a message is sent, a copy is saved in the Sent folder for reference. However, if the mailbox has exceeded its quota, there is no more storage space available to save the sent message.
To determine an account's disk usage and see if it is approaching or has exceeded its quota, most email client applications provide information on current disk usage. For example, the Roundcube webmail application displays quota information in the bottom-left corner, while Mozilla Thunderbird displays it in the bottom-right corner.
If an email account consistently exceeds its quota, there are two options. The first option is to increase the account's quota or remove it altogether. This can be done in cPanel, where you can set the quota information. The second option is to instruct the account owner to regularly review and delete old messages to ensure that the account stays within the quota.
When an account exceeds its quota, Roundcube visually indicates this by displaying a warning or error message. This alert serves as a reminder to the user that their mailbox is full or close to reaching its limit. The image below shows: 
By monitoring and managing the account's quota, users can avoid any disruptions in receiving or sending emails due to exceeding the storage limit.
