SMTP error "451 4.7.1 Please try again later"

If the DEFENDO mail server keeps rejecting emails with the message "451 4.7.1 Please try again later", it is usually caused by either the mail virusscan module or the attachment filter. It is often sufficient to stop and then start the service "Mail server" in the DEFENDO menu "Monitoring -> Services" on tab "Server".

If this solved the problem for the moment, but the same situation reappears after a while, it could be caused by memory shortage due to the SPAM filter being enabled twice. Please check if both, the relay SPAM filter and the userspecific SPAM filter are running. If this is the case, you should disable one of them. You will find more information in the article >Configuring and optimizing the SPAM filter.