We’ve noticed that some of our customers are using “Sender Verify Callout” on their mail servers, which is a system which takes the sender’s email address of an incoming message and tries to verify that it’s a valid email address by going to the mail server of the sender’s domain and doing an SMTP transaction to verify it.
While normally this would not be an issue, there is a situation where using this process on your mail servers that are protected by Easy Antispam can cause problems.
Here’s the situation:
1. A spam message slips through our filters (while we try to block them all, some do get through)
2. Our gateway attempts to send the message to your server
3. Your server does “sender verify callout” on the sender’s email address; it fails
4. Your server issues a 550 response rejecting the message
5. Our gateway, because your server rejected the message, thinks that the “to” address doesn’t exist, because it’s the same error number that’s used in that situation
6. Our gateway caches the state of the “to” email address for 1 hour, rejecting all mail to that user during that time period
If you’re seeing periodic, sporadic situations where mail to a specific user is being rejected for about an hour, this is probably the reason why. This is why it’s a bad idea to use “sender verify callout” with Easy Antispam.