I just analyzed the email headers and notice that when sending out emails on Exchange 2013, the recipient receives the email via Microsoft SMTP Server (TLS) and not ESMTP. It says it in the email headers and there are some delays.
When I send out emails in Exchange 2010, the recipient receives the email via ESMTP.
When using Exchange 2013, the first email to a new domain takes around 30 seconds for the recipient to get it and subsequent emails takes likes 0 second. It's immediate on subsequent emails. If I haven't emailed that domain for a while again, I'm back to
square one. First email takes around 30secs to arrive in recipient's mailbox and 0 seconds or immediately for subsequent emails right after I send the first one.
For example, Exchange 2013 sends first email to test@hotmail.com (30 seconds. takes some time to arrive in the recipient's mailbox). Exchange 2013 sends second email to test@hotmail.com (0 seconds or arriving immediately in the recipient's mailbox).
For example, Exchange 2010 sends first email to test@hotmail.com (0 seconds. arrive in the recipient's mailbox immediately). Exchange 2010 sends second email to test@hotmail.com (0 seconds or arriving immediately in the recipient's mailbox).
The only difference I notice is the email headers for Exchange 2013 show 'Microsoft SMTP Server (TLS)' and Exchange 2010 show 'ESMTP'.
It seems like Exchange 2013 first tries to send out the email via TLS but the remote server doesn't support it so it falls back to sending out the email without TLS thus the ~30 second delay.
How do I fix this? How do I turn off TLS for SMTP on Exchange 2013 so I can do another test with TLS off and see if that's the culprit?