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What does SMTP protocol violation: synchronization error (next input sent too soon) mean?

April 26, 2017Error input mean Protocol sent SMTP Soon synchronization violation

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What does SMTP protocol violation: synchronization error (next input sent too soon) mean?

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A0086: SMTP is a lock-step protocol, which means that, at certain points in the protocol, the client must wait for the server to respond before sending more data. Exim checks for correct behaviour, and issues this error if the client sends data too soon. This protects against malefactious clients who send a bunch of SMTP commands (usually to transmit spam) without waiting for any replies. This error is also provoked if the client is trying to start up a TLS session immediately on connection, without using the STARTTLS command. See Q1707 for a discussion of this case.