Updates to default Anti-spam settings on SmarterMail.
Due to the number of customers having issues with emails being rejected or going missing due to Greylisting we have now disabled this anti-spam measure by default for all new and existing users on our SmarterMail server.
Greylisting is effective because many mass email tools used by spammers do not queue and reattempt mail delivery as is normal for a regular Mail Transport Agent. They do not queue and resend email as this requires the expenditure of resources (spamming normally operates on very narrow margins). Since the advent of Greylisting spammers have taken to rerunning their mail delivery tools to resend the same email again without having to expend resources queuing the email. This approach still requires them to expend additional resources resending the email the second time though. As of 2011 spammers have been using this technique for many years.
Delaying delivery also gives real-time blackhole lists and similar lists time to identify and flag the spam source. Thus, these subsequent attempts are more likely to be detected as spam by other mechanisms than they were before the Greylisting delay.
The reason for the issues some customers have experienced is because some SMTP clients and SMTP servers acting as clients may interpret the temporary rejection caused by Greylisting as a permanent failure. Old clients conforming only to the obsolete specification (RFC 821) and ignoring its recommendations may give up on delivery after the first failed attempt—RFC 821 states that clients "should" retry messages rather than using the word "must". RFC 2119 dictates that "should" means recommended and to ignore at your own risk, and it is a violation of the current SMTP standard for the client to fail to retry. The current SMTP specification (RFC 5321) clearly states that "the SMTP client retains responsibility for delivery of that message" (section 4.2.5) and "mail that cannot be transmitted immediately MUST be queued and periodically retried by the sender." (section 188.8.131.52).
If you still wish to use Greylisting then you can re-enable it via the webmail interface. We also have a selection of SmarterMail video tutorials on our support helpdesk HERE if you need help configuring your optional email settings and spam filters.