You shouldn't need to restart MailService.exe, you can kill MailService_SubProcess.exe if you want to "reset" Cyren, MailService will automatically boot it back up after ~30 seconds or so and if it needs to perform a scan.
There should also be a SubProcessConfig.xml near your mailConfig.xml file, in "C:\Program Files (x86)\SmarterTools\SmarterMail\Service\"

In that file, you can set logging to "True" and you'll get logging describing what Cyren is doing, this log is not like other logs, it goes into that same folder under the name of "SP_Log.txt".

Our Cyren hasn't worked in weeks, clients are losing their minds. Have tried all the support referencing info i could find, nothing seems to fix the issue, it just identifies 2 or 3 emails a day while in the past it would trash hundreds of them per day.

Following the instructions above we have a log that's been running for a couple of weeks but all we see are these things, not sure if we're meant to have more than that.  Restarting the server or the service has had no effect (exhibited by no changes in the SmarterMail Cyren Antispam reporting graphs) 

Who provides support for the Cyren plugin?  Should we contact them or is this entirely on Smartermail to support their integration?

00:25:49 Cyren connection string: ServerAddress=resolver%d.stools.ctmail.com;LicenseKey=(hidden):SmarterMail-16.3.6970
00:25:49 Connecting to Cyren using: resolver%d.stools.ctmail.com
00:25:53 Connecting to Cyren was Successful.
09:50:01 Reading Message from Stream...
09:50:01 Scanning Message: 64690
09:50:02 Sent Result(64690:Bulk|Unknown) back to server
13:00:15 Reading Message from Stream...
13:00:15 Scanning Message: 65208
13:00:16 Sent Result(65208:Unknown|Unknown) back to server
13:10:33 Reading Message from Stream...
13:10:33 Scanning Message: 65246
13:10:33 Sent Result(65246:Unknown|Unknown) back to server
13:20:20 Reading Message from Stream...
13:20:20 Scanning Message: 65262
13:20:20 Sent Result(65262:Unknown|Unknown) back to server
13:30:25 Reading Message from Stream...
13:30:25 Scanning Message: 65293
13:30:25 Sent Result(65293:Unknown|Unknown) back to server

I completely agree.  Cyren has about a 1% effectiveness and I am seeing it stop from time to time too.  Lots of problems getting through.  Phishing emails are way up, literally everything.  I questioned this with Support and they looked at my server and think everything is running okay.  I agree, it looks okay.  But the volume of spam getting through is driving my users batty.

Cyren was garbage when we used it. It would miss identify emails that my cat could have flagged as spam. Though, to be fair, MessageSniffer is not much better... Cyren had issues with their DC going down which would stall out mail, SM crashes etc. drove us away from Cyren to MessageSniffer.

We have a pop3 mailbox setup to send false negative spam emails to that Arm Research pulls down, I think it is just all for show, they pull down the emails, but nothing ever happens. I've been getting the same non-english spam mail for over a year now, reported every one of them, makes no difference.

We used to use an external SpamAssassin appliance that worked pretty well, but since SM took away the spam/ham folders, we cannot train SA anymore, which stinks :(

We also thought Cyren (previously ComTouch) DC was sometimes down, but that is not correct. 

Cyren plugin for SM sometimes fails until you kill/restart MailService_SubProcess.exe. In the meantime our others spamassassin and rspamd incoming gateways for other mail systems still scans mails sucessfully using their respective cyren plugins and licenses.

I posted about this a few months ago, but no reaction from ST team.

edit: I found the post I wrote about it (under our cto account)

We also have problems with the Cyren plugin failing. Messages get stuck in the spool with the Status = Spam Check. Killing MailService_SubProcess.exe fixes the problem and the spool clears out.

Running SM 15.x