Outlook 2016 freezes when sending SMIME signed messages

Question

I have an Outlook 2016 instance linked to a Microsoft Exchange 2013 server, running on a Windows 2016 Standard Remote Desktop Server. Everything works fine, just when I send a S/MIME signed message, Outlook freezes for 4-5 seconds ("not responding") before it proceeds to send the message. The mail is sent perfectly fine, and no other errors occur. Sending the same message to the same recipient without signing it works in the way expected, after clicking on "send mail" the window just disappears within a second and the mail is sent.

I am using a free SMIME certificate from Comodo, but have the same problem with a certificate from StartSSL (now StartComCA). I dont think the source of the certificate should matter here.

I have already tried setting different HASH-Algorithms for the message signing (SHA256, 384 and 512, since I refuse to fall back to SHA1). I have disabled all loaded Outlook Add-Ins via /safe-mode, the only third party Add-In worth mentioning is the ESET File Security Plugin, everything else is stuff from Microsoft.

What is the best way to debug this behaviour?

Answer 1

Turns out, we secured the server a little too much. The certificates use a revocation list from comodo (obviuosly) that is checked via an http-URL. The Server was behind a proxy that did not allow the Revocation List to be checked, and so it timed out after a few seconds.

After correcting the proxy settings, everything worked.