I came across this same issue today.  I also increased maxJsonLength in web.config to no avail.  It's only on one email and I've never encountered it before on any SM version over the past 7-8 years.

-John

You can set the MaxJsonLength property on your web.config:

<configuration>
   <system.web.extensions>
      <scripting>
         <webServices>
            <jsonSerialization maxJsonLength="50000000"/>
         </webServices>
      </scripting>
   </system.web.extensions>
</configuration>

That is indeed how it is set.  Both CL Lee and I did this, as alluded to in the posts.

My solution: I had the client have his client re-send the email.  It had an attachment, so I told him to have it resent zipped up.  Since this has only happened once in nearly a decade, I'm not too concerned anymore.

Thanks for your update, secretwep. 

C.L Lee, please also note that you'll need to stop the SmarterMail Service before modifying system files. 

We received on email with 10mb of attachents.

I tried to do this in the web.config, restarted the IIS and MailServer and the problem continues.

Using version 15.7.6726 

Any tips?

@Cleber I believe you'll need your own thread on this one, as this post refers to later releases, and your issue looks to be related to maximum message size rather than a JSON problem.

Hello @Kyle. I will open a support ticket with more information. I assigned with that thread because the message error its the same. Another messages with attachs with same or higer size works fine. It's for a particular message.

Thanks Cleber, that sounds like a great plan. We'll get to the bottom of it for you!