If the third retry call does not succeed, perform another set of retries after 20 minutes.
These codes are always in pairs, which means both servers transmit the codes until either the conversation is successful, or fails.The IP address is automatically removed from the block list after five minutes.Continued invalid connections result in the IP being readded to the block list.By default, Mimecast only accepts 100 RCPT TO entries per message body (DATA).The error triggers the sending mail server to provide the DATA for the first 100 recipients before it provides the next batch of RCPT TO entries. Most mail servers respect the transient error and treat it as a "truncation request".You should retry SMTP requests that receive 4xx errors.In this case, to reduce the likelihood of generating duplicates, we recommend that you implement an exponential retry method with progressively longer waits (5, 10, and 30 seconds) between consecutive timeouts.The remaining two numbers in a code provide information on the reason for the failure.The code types are: Both the sender AND recipient domains specified in the transmission are external to Mimecast, and aren't allowed to relay through the Mimecast service and / or the connecting IP address isn't recognized as authorized.However, there are things that are likely to be true, and we are going to talk about them. The most likely reason for this error is that recipients’ spam filters didn’t let your message through.Well, you’ve received this nasty “554 5.7.1” SMTP error message—maybe even more than one. There are a couple of things that might have led them to reject your message: Most of blacklists allow you to remove an IP if it was added due to a mistake or misconfiguration. However, if your email gets on the same blacklist again, it might be close to impossible to remove it a second time.