HOTSPOT
Your company has an Exchange Server 2013 organization.
You configure domain security with a partner organization.
You configure the required connectors.
You plan to verify whether the partner organization configured the required settings for domain
security.
You enable logging for the Send connectors and the Receive connectors.
You need to verify that the STARTTLS command is issued by an Exchange server when an email
message is sent to the partner organization.
Which log folder should you review? (To answer, select the appropriate folder in the answer area.)
Answer: 
how about protocol log instad of HUP
Protocol Log does make sense.
https://technet.microsoft.com/en-us/library/bb124531%28v=exchg.150%29.aspx
Talks about turning it on for send and receive connectors.
Same here…
http://social.technet.microsoft.com/wiki/contents/articles/17842.configuring-domain-security-on-exchange-server-2013.aspx
Easiest way to test this is to just send email from one organization to another from Outlook. If you get the message with green check mark, you are all set. If not, then you’ll need some troubleshooting. You can enable protocol logging by executing :
Set-ReceiveConnector Internet -ProtocolLoggingLevel Verbose, and
Set-SendConnector Internet -ProtocolLoggingLevel Verbose
to verify TLS channel. If message doesn’t arrive to recipient but doesn’t come back as NDR, you should check queue.
I think HUB is actually correct from the following URL: technet.microsoft.com/en-us/library/aa997624(v=exchg.150).aspx which lists the following:
Receive connector protocol log files for the Transport service on Mailbox servers %ExchangeInstallPath%TransportRoles\Logs\Hub\ProtocolLog\SmtpReceive
Receive connector protocol log files for the Mailbox Transport service on Mailbox servers %ExchangeInstallPath%TransportRoles\Logs\Mailbox\ProtocolLog\SmtpReceive
Receive connector protocol log files for the Front End Transport service on Client Access servers %ExchangeInstallPath%TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpReceive
Send connector protocol log files for the Transport service on Mailbox servers %ExchangeInstallPath%TransportRoles\Logs\Hub\ProtocolLog\SmtpSend
Send connector protocol log files for the Mailbox Transport service on Mailbox servers %ExchangeInstallPath%TransportRoles\Logs\Mailbox\ProtocolLog\SmtpSend
Send connector protocol log files for the Front End Transport service on Client Access servers %ExchangeInstallPath%TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpSend
This is a very tricky question.. although the folder for send and receive connector logs is indeed “Protocol Log”. The file path for those logs is
C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\HUB\ProtocolLog
The file path “C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\ProtocolLog” is for HTTP Client logs
agree
“The transport component on the sender Mailbox server initiates a mutual TLS session with the transport component on the target Mailbox server by exchanging and verifying their certificates.”
Talking Transport Components – it sounds like HUB-to-HUB communication. So logs should be under HUB folder.