When you implement greylisting on your mailserver you can have a problem to send e-mails to other mail servers which implement Sender Address Verification (SAV). You will see logs such as:
Mar 17 14:17:49 ns2 postfix/smtp: 45D49241802: to=<firstname.lastname@example.org>, relay=receiver.server.com[X.Y.Z.Q]:25, delay=4669, delays=4654/0/0.3/15, dsn=4.1.7, status=deferred (host receiver.server.com[X.Y.Z.Q] said: 450 4.1.7 <email@example.com>: Sender address rejected: unverified address: host your.server.com[A.B.C.D] said: 450 4.7.1 <firstname.lastname@example.org>: Recipient address rejected: Policy Rejection- Please try later. (in reply to RCPT TO command) (in reply to RCPT TO command))
You can simply see all servers which require SAV from the logs of your mailserver (in my case Postfix):
# cat /var/log/mail/info | grep "Sender address rejected: unverified address"
The remote mail server verifies the sender address by connecting to your mail server and imitating a initialization of an SMTP connection:
Mar 17 14:04:11 ns2 policyd: rcpt=422745, greylist=new, host=receiver.server.com (X.Y.Z.Q), email@example.com, firstname.lastname@example.org, size=24
There are several solutions:
- Parse your log and update your Whitelist database;
- Delay the DATA command – Do greylisting reject after your server says OK to RCPT TO command.
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