I am facing the same dilemma.
Upgraded to 15.7.6390
Every email sent from the server is stuck.
I had a SmarterTools Tech connected to it today and he asked me to restart the server and I have not seen nor heard from him since!
My errors are a bit different.
First thing I notice is the the "Next Attempt" is set to when it was sent. Which makes no since should I just wait 24 hours and see if it goes? I tried resetting the Server Clock and mine but all the mail sits in the spool. Sometime I can force the delivery sometimes it will not budge.
Mine also seems to be related to the TLS handshake. The mail transaction seem to break down at that point. It will negotiate on IP4 then try switching to iP6 then just crash.
Any help would be appreciated...
Delivery Log Snippet:
--------------------------
2017.07.06] 01:17:33 [30002] Initiating connection to 98.136.216.25
[2017.07.06] 01:17:33 [30002] Connecting to 98.136.216.25:25 (Id: 14)
[2017.07.06] 01:17:33 [30002] Binding to local IP 68.15.157.104:0 (Id: 14)
[2017.07.06] 01:17:33 [30002] Connection to 98.136.216.25:25 from 68.15.157.104:1135 succeeded (Id: 14)
[2017.07.06] 01:17:34 [30002] RSP: 220 mta1167.mail.gq1.yahoo.com ESMTP ready
[2017.07.06] 01:17:34 [30002] CMD: EHLO smtp.medstaffingsolutionsusa.com
[2017.07.06] 01:17:34 [30002] RSP: 250-mta1167.mail.gq1.yahoo.com
[2017.07.06] 01:17:34 [30002] RSP: 250-PIPELINING
[2017.07.06] 01:17:34 [30002] RSP: 250-SIZE 41943040
[2017.07.06] 01:17:34 [30002] RSP: 250-8BITMIME
[2017.07.06] 01:17:34 [30002] RSP: 250 STARTTLS
[2017.07.06] 01:17:34 [30002] CMD: STARTTLS
[2017.07.06] 01:17:34 [30002] RSP: 220 Start TLS
[2017.07.06] 01:17:48 [30003] Exception: Authentication failed because the remote party has closed the transport stream.
[2017.07.06] at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
[2017.07.06] at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
[2017.07.06] at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
[2017.07.06] at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
[2017.07.06] at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
[2017.07.06] at RelayServer.Clients.SMTP.ClientConnectionSync.InitiateSsl(Boolean validateAllCerts)
[2017.07.06] at RelayServer.Clients.SMTP.SmtpClientSession.#VG(Boolean #5Ni)