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

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 a client unexpectedly tries to start up a TLS session immediately on connection, without using the STARTTLS command. See `FAQ/Encryption_TLSSSL/Q1707`_ for a discussion of this case.

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