If you use Zimbra and cannot receive email while sending works normally, check the log for a message like the following.

zimbramail postfix/smtpd[xxx]: NOQUEUE: reject: RCPT from mailx.ezylinux.com [58.137.249.xx]: 554 5.7.1 Service unavailable; Client host [58.137.249.xxx] blocked using multi.uribl.com; Too many queries from 173.194.97.xx. Emails should not be blocked based on this response. Recipient should fix their broken implementation.

Cause The mail system has made too many blacklist queries to URIBL, causing URLBL to block its IP. The system can no longer query URIBL, resulting in this error. Solution Disable DNS queries to URLBL by editing /opt/zimbra/conf/spamassassin/50_scores.cf and finding:

score URIBL_GREY 0.25

score URIBL_RED 0 0.001 0 0.001 score URIBL_BLOCKED 0 0.001 0 0.001 score URIBL_DBL_ERROR 0 0.001 0 0.001 Change it as shown below: score URIBL_BLACK 0 score URIBL_RED 0 score URIBL_GREY 0 score URIBL_BLOCKED 0