Block cracking issues


Verified User
Dec 24, 2012
Just to share my experiences with using block cracking for the past few months since switching to using custombuild 2 on one of our servers.

In most cases, it does not appear to cause any problems. However, for certain not-too-improbable scenarios, it becomes an issue. So much so that we've decided it was better from both our and our clients' perspectives to use more proactive send quota limits and deal with the rare hacked account than to continue using block cracking.

1. Emails to multiple recipients
As block cracking does callout verification to the receiving server for every recipient to try to catch failed ones, this adds delay at SMTP connection time that increases linearly with every recipient. Given that some/many mail servers do tar-pitting to slow down potential spammers, this can add anything from 5~30 seconds PER recipient. So if your customer needs to send out a mass email to their all customers, their email client program can literally get locked up sending that email for hours.

2. Recipient servers with multiple MX
In a similar fashion, if any of the recipient servers has an extensive list of MX servers in their DNS records, exim will spend a fair bit of time going through them to determine which to use. While this is faster than the actual callout, with some large corps, the list of MX is big enough to make it just as bad. For some reason, Outlook will choke on this situation, exim will log an unexpected disconnection while the message get stuck in the user's outbox.

Hope it helps anybody who is running into unexplainable/random complaints to their support department on a server that appears to be running smoothly otherwise.