Call out verification cache

If you have just set-up a new email address and our servers are rejecting it with a Non Delivery Report saying email does not exist, then this is almost certainly because our MX servers have previously received an email for that same address and have cached the lookup result that it did not exist. The cache lifetime can be up to 60 minutes. 

When any incoming email arrives, before processing it further, our MX servers perform something named ‘call out verification’ which involves connecting to the destination server to see whether it will accept the email – this happens whether that destination is Zimbra or your in-house server (if you are using Email Security).  This lookup result is cached for up to 60 minutes.

A common scenario is where an administrator sends a test email for an address which does not exist – this generates a bounce and the lookup result is cached by the MX servers. Realising the mistake, the administrator creates the email address and sends another test email – but because of the previously cached lookup, the same error is returned.

How to fix call out verification cache issues

The cache automatically clears after no more than 1 hour.

Cache lifetime is not exactly 60 minutes due to various factors, but it will not be longer than 60 minutes, usually less.

Useful Links:

• Why are emails to new email addresses I just set-up still bouncing?
• The Email Bounced Back to the Sender
• No Bounce Back to Sender, but Email hasn’t Arrived
• How to Flush your Local DNS Resolver's Cache