Email stuck in Exchange Message queue: SMTPSEND.SuspiciousRemoteServerError; remote server disconnected abruptly – returned 451 4.4.0

After migrating on new Exchange environment I have noticed that couple of emails are stuck in Message queue with following error:

 

SMTPSEND.SuspiciousRemoteServerError; remote server disconnected abruptly – returned 451 4.4.0

 

After some troubleshooting, we found out that our Firewall had IPS feature enabled on SMTP traffic. Once disabled, emails successfully went through.

The HTTP service located at https://sfbfrontendserver01/liverserver/userpinmanagement/fabricmanagement is unavailable

After fresh Skype for Business 2015 frontend pool installation, I was not able to start RTCSRV service, I tried different things and I ended up reseting pool with command Reset-CsPoolRegistrarState. Command failed with following error:

 

After some troubleshooting I noticed that I forgot to install latest Cumulative Update.

I downloaded Skype Server Update installer from:

https://www.microsoft.com/en-us/download/details.aspx?id=47690

followed following guideline:

https://support.microsoft.com/en-us/kb/3061064

and installed CU on all FrontEnd servers.

After reboot all CS services started successfully.