Introduction
There's no point accepting mail if it is to a recipient email address that doesn't actually exist on your destin ation mail server. Therefore the servers check the destin ation servers first to check if a recipient email address is an existing email account for which the destin ation server accepts email. The filtering systems internally keep track of existing/non-existing email accounts at the destin ation server to minimize the number of recipient callouts.
5xx destin ation server rejects
Whenever the destin ation server permanently rejects the email for a certain recipient with a 5xx error code, the destin ation address is considered invalid and all email sent to the recipient address will be rejected. This information is cached locally on each filtering server separately for up to 2 hours. You can clear the callout cache using the webinterface or API.
Technical details
The filtering servers do a 'null sender' callout. This means that the filtering server will connect to your destin ation server, using a sender of "<>" (just as a bounce message does), and the real recipient. After checking the recipient, the connection will be properly closed (i.e. no message is actually sent). You need to ensure that your destin ation servers will correctly verify addresses in this fashion.
