A client of mine keeps receiving the following email when sending mail but their emails are sent successfully.

Your outgoing (SMTP) e-mail server has reported an internal error... 
The server responded: 451 qq read error (#4.3.0)

In the mail log (/usr/local/psa/var/log/maillog) I receive the following error:

/var/qmail/bin/relaylock[3152]: /var/qmail/bin/relaylock

My SMTP Service is setup as followed, if this helps

service smtp
{
    socket_type     = stream
    protocol        = tcp
    wait            = no
    disable         = no
    user            = root
    instances       = UNLIMITED
    env             = SMTPAUTH=1
    server          = /var/qmail/bin/tcp-env
    server_args     = -Rt0  /var/qmail/bin/relaylock /var/qmail/bin/qmail-smtpd /var/qmail/bin/smtp_auth /var/qmail/bin/true /var/qmail/bin/cmd5checkpw /var/qmail/bin/true
}