I use Declude on my incoming gateway, so stopping the Decludeproc service will cause messages to build up in the spol\proc folder waiting for Declude to act.

That may not be an option for you 

How your gateway is configured? Backup MX? 

EDIT: On my end incoming gateways works for years as Domain Forward (since main server is not present in domain MX configuration) with Domain verification enabled (All Web Service) where SmarterMail Gateway mode is enabled.

I am also using the gateway as MX with Domain Forwarding.    

Declude and the "Enable spool proc folder" option work like this:   Incoming messages are dropped into the spool\proc folder for Declude or another product to process.   The other product indicates completion by moving the messages into the spool folder, where SmarterMail picks them up and proceeds with delivery.   The other process can delete messages for silent discard, and SmarterMail doesn't care.  Each message is represented with two files, a <message#>. EML file of the message and a <message#>.HDR file with message metadata.

With Declude, I just stop the Declude process so messages build up waiting for Declude to process them.

Without Declude, you should be able simulate the same result:

- Stop the SMTP service briefly

- Enable the spool\proc feature

- Start the SMTP service to start delivering messages to the dead end

- Fix your downstream problem

- Stop the SMTP service again

- Drag all files from spool\proc to spool.   Or move a few at a time, being careful to keep EML and HDR pairs together.

- Start the SMTP service again to process the delayed messages.

Hopefully SmarterTools staff will review and comment, in case if I have overlooked something.

If not, open a ticket to have the concept reviewed.