"Explicit" is to be understood as asking for STARTTLS on OSI Level 7 instead of Level 4. If the client cannot establish an encrypted connection, the attempt is

"Explicit" is to be understood as asking for STARTTLS on OSI Level 7 instead of Level 4. If the client cannot establish an encrypted connection, the attempt is aborted in the first place. Of course, there is the hazard as in HTTPS that an MITM attacker might somehow cause a downgrade to the unencrypted protocol variant.