Today, I recycled one of my host instances, and it didn’t start. Then I tried all of them, and several had issues. Finally, it dawned on me that I had changed the 64-bit config file:
“c:Program Files (x86)Microsoft BizTalk Server 2010BTSNTSvc64.exe.config”. The 32-bit host-instances were all starting fine, because it uses a different config file (BTSNTSvc.exe.config).

Check your web config file for malformed xml. (For example, make a copy called BTSNTSvc64.xml and open it in Internet Explorer to quickly find the error.)

I had something like this (to mailSettings close tags).

<code>  
<system.net>
    <mailSettings>
      <smtp from="ClientSupport@someone.com">
        <network host="newaddress.something.com" password="" userName="" />
        <!-- formerly was: <network host="10.1.1.1" password="" userName="" /> --> 
      </smtp>
    </mailSettings>    </mailSettings> 
  </system.net>
</code>

That’s it, just fix your web.config hopefully.