Unable to send an email via webmail or configure mail client: “The maximum number of sent messages is exceeded” or “Insufficient System Storage” or “The limit on the number of allowed outgoing messages was exceeded”

Symptoms

• Email sending fails:

  • In MailEnable webmail:

    An Outgoing server (SMTP) error occurred while sending mail.
    The server responded:
    Requested action aborted:
    The maximum number of sent messages is exceeded.

  • In Horde webmail:

    There was an error sending your message:
    Insufficient System Storage.

  • In the Roundcube webmail:

    SMTP Error: [554] 5.7.0 Your message could not…